When Reid first met the BAU team he nearly had a heart attack, he thought his wrist had been set on fire, and he knew it was Morgan that set it off. Boy was he glad he had long sleeves on that day. It gave him time to assess the situation, and ultimately come to the decision that Derek Morgan would never see his mark, no matter what happened.
The drop-dead gorgeous milk chocolate hunk did not need to be tied down to skinny little Spencer Reid. Morgan would probably resent him if he knew, because, seriously, he got the short end of the stick when it comes to soul mates.
Morgan is handsome, muscular, charming, witty, a social butterfly and not to mention TOTALLY STRAIGHT. How would he even handle having a male soul mate, especially with his past with Buford? He would simply hate Reid for it, resent him totally, it would ruin their working relationship and their friendship. A friendship Reid treasures so much, it's the closest he's ever come to having a best friend, and he wants to keep that friendship, he wants to keep it more than almost anything else. But the thing Spencer wants the most is impossible the odds are 87 million to 1 (and Reid has done the maths), for Morgan to love him back, but that just won't happen, its better off to live this way, protecting Morgan, protecting the team too, and protecting his fragile heart.
Spencer Reid is committed to living half a life, knowing but never having his soul mate, it is the saddest existence known to man, but he must for the sake of Derek Morgan.