Because people are complicated, and people rarely do. He can rationally envision and plot a course of action that'd ideally work as well for him as it would for anyone else, so it's not bad advice, but he himself is entirely consumed by the perfect vision of how things ought to be and ought to have been.
Which is, having a high-school sweetie, being each others' first, settling since youth for the long haul through thick and thin. He resents the people whose responsibility it was (his parents) for not equipping him since youth with eyes that would have realised that for him as a natural development of coming to manhood. I mean, he loves them, but he mocks them, doesn't respect them, because I think he feels they failed him in various ways, enabled the follies of his youth, and on that front failed in their own lives by splitting up.
But unlike being set on standards with architecture and furniture like Nick is, every day moves him away from the thing he wants to have wanted when it was time for it, when it was time to make such deep, sincere and lasting conncetions, and any attempt at a relationship further on becomes an ever increasingly grotesque reminder how things should not be rather than a chance to put to work in his own life what he has realised to be a fundamental good to have. The more strongly you're to able conceptualise your own ideals, the more you're captured by them. In that respect, he is an advice machine for the same reasons he can't apply them in his own life. And he often mocks tendencies in others he has realised he is or has been held back by - like waiting for the "perfect moment" to start a venture, be it accreditation or picking up exercise.