Dom knows he isn't the one Elijah really wants. He's seen the way Lij looks at Sean when he thinks nobody's watching: those two are Frodo and Sam in more ways than one.
But Sean is married, and Lij adores Christine and Ali so much he'd never risk hurting them, even if Sean returned his feelings. So he just watches, and recently, he has turned to Dom.
Dom knows he isn't the one Elijah really wants. But he wants (and sometimes he thinks he loves) Elijah, and if he can only have him on these terms, then so be it.