He knows he shouldn’t. He knows that he can’t, because what if anything happens to Seb? What will he do then? He told himself he wouldn’t love anyone, only to have a fascination. But this is far more than his Holmes fascination, this is something he wishes he didn’t have. For the first time, Jim feels. For the first time, Jim feels love.
The way I see it, every life is a pile of good things and bad things. The good things don’t always soften the bad things, but vice versa, the bad things don’t always spoil the good things and make them unimportant.