Heidi and Stan's relationship would be so great because they also complement eachother. But also share similarities.

One Similarity the both love animals. How the complement each other through that:

One example would be if they tried to save some animals. Heidi would keep him grounded and not go over board immediately and not act on his emotions. With her empathy she would make him talk about his feelings or hear him out when he's frustrated and giving him the support and strength he needs.

Meanwhile Stan would push her to do something and seeing something being achieved. He would get her more motivated.

Another similarity is that they are both emotional people. Stan is aswell but he is more hiding it. How the complement eachother in that:

Stan is rather pessimistic and a skeptic meanwhile Heidi is more an optimist and naive. With them balancing eachother out Stan could be more positive, and Heidi be less prone to get manipulated.

Another similarity is that they both lost their friends for a long period of time.

Stan during his depression and Heidi when she dated Cartman.

They both share what they have learned during that time.

Both of them lost something (their friends).

Both got really down from it.

Both tried to get comfort in the toxic thing (Heidi: Cartman Stan: alcohol.)

Both felt good for a while and after realizing it wasn't good for them.

Both had a speech at the end.

Both of them would definitely understand each others story best and empathize with them.

Stan feels alot he is an emotional person but he has learned to shut his feelings down because others can't deal with them.

Heidi is an empathic person. Just her speech in Put It Down.

She wouldn't stress Stan to get better and try to fix him like Kyle and Wendy. She would be just there for him. And that's what he needs. Not someone who guilt trips him for having strong emotions. Heidi is also quite impressionable and Stan is rather skeptical.

He could help her with that.

And she could also help him see life more colorful.

And Heidi is vegan. And Stan loves animals. Heidi could show him some vegan alternatives.

Now this is an answer I found on tumblr. It's from a deactivated Tumble account named Stan Marsh.

Someone asked why they shipped Steidi:

First of all, let's just focus on stan as a character. (not only said by me but by matt n trey too): stan, throughout the show, has been shown to be extremely empathetic and compassionate as a kid. (especially towards animals). he's easily hurt by others, and he feels his emotions on a deeper level. so a lot of the time, his emotional complexity gets in the way of his sensibility, and he doesn't know how to cope. when his intense feelings get in the way, he resorts to what he thinks is best to do– which nearly always isn't the best decision because he doesnt know how else to deal with whatever he's experiencing.

take s7 ep14 "Raisins", as an example:

because of stan's heavy emotional insights that effect him deeply, he often gives up easily (unless its something he wants to find answers to) causing him to be typed as the "weak" and "pussy" character of the show. whether it be by the fandom or by his own friends.

s5 ep13: "kenny dies"

example: stan completely walked away from seeing kenny in the hospital because it was way too hard on him to see a friend in that state

i believe because of him being portrayed as a weakling, no one wants to understand the pain from his perspective– that's why he doesnt know how to emotionally deal with all the crap he's put through… no one cares enough to help because his emotions are "too much to deal with".

take a look at stan in s15 ep8, "assburgers," the most prominent example of literally no one being there for him when he falls into a major depression,

all of this leads up to stan being the cynical pessimist of the group, seeing as no one is there for him, and the fact that he feels too much.

and when his negative thoughts take over, along with the fact that he's nearly always gets hurt by others, it connects back to what i mentioned before– it connects back to the fact that he copes with everything around him in the wrong way. he stays in this state and he can't get out of it. his emotions take over and he sort of gives up.

now this… this is where heidi comes in. but first, heidi's character alone (only pertaining to s20-beginning s21 heidi):

despite the fact that heidi usually tries to help certain situations, she can also be portrayed as a weak, ignorant character given the fact that she's extremely impressionable. and while this is remotely true, heidi has been seen too, in the show, as a compassionate and empathetic character towards either animals, or if its the passion for fixing her abusive boyfriend. shes constantly hoping to fix whatever's wrong. showing how she sees the potential and hope in whoever's around her.

s20 ep5: "Douche and a Danish". she wanted to bring the girls and boys together, and organized a whole bake sale.

she has often been seen as trying to get cartman, her dead end boyfriend, to open up and explain to him what's wrong. always trying to fix their relationship, because she's a character who sees the good in people. and because of her persistency, she's been through so much. with cartman abusing her, and her friends unwilling to listen to her. then, her constant trying to fix and do often leads her to be miserable.

so all in all, heidi feels deeply for people. she hopes to see the good in others, and usually tries to change things for the better. she has passion. and empathy. and even if she is a weakling– she's persistent and always wants to do whats right. she holds on instead of giving up too quickly, which sometimes leads her to her own demise, leaving her in pain, not unlike: stan.

now this is where the steidi dynamic comes in.

as mentioned, stan usually doesnt know how to cope with overwhelming emotions, and he remains in a sort of "phase" the wrong way. heidi would act as en emotional guide for him in a sense where she listens to him. with that, stan would be happier than he's ever been.

she'd be able to understand him, unlike most people– because both of them have heightened emotions. so along with their similar emotional complexities, the two of them as individuals have been through too much. too much for 10 year olds to handle. and so they'd be able to relate with feelings, feeling a mutual compassion for each other– while also being able to connect through events in their life.

stan being unable to deal with his emotions in the right way, and heidi pulling him out of his pessimistic, depressed state.

considering she'd be able to find the good in him that stan needs to hear

relating: with stan being abandoned by his "best friend" in a brief period of his life, and heidi's friends who won't listen to her.

heidi would be the one to hear him out, and he would definitely hear her out as they're both good listeners

both of them, as seen, have emotions as deep as the ocean, they'd find feelings in each other they didn't know they needed.

they'd keep each other grounded, heidi would never let stan give up on himself and stan would provide her with the reflecting empathy and love she deserves

with heidi's impressionable state of being, stan would definitely be the one to help teach her how not to let people easily manipulate her

seeing as they both have feelings of intense consideration towards animals, just imagine them banding together for animal rights !

just… steidi. pls consider it.