Bernie Sanders - Ben Garrison's depiction of Bernie's endorsement of Hillary.

Well, let's look at the alternatives:
If he doesn't endorse Hillary (or worse – if he actively goes against her), then he's effectively supporting Trump.
If he doesn't stick close to her somehow, then he has no way of advising her, pressuring her, or influencing her in any way whatsoever.
I'm sure he is just as upset over having to support her as everyone else, but this way we at least have a better chance at keeping Trump away from the White House (and possibly even getting some good policies through, although I admit that chance is relatively slim).

Because unlike all those other lying politicians, he's actually stuck to his guns for decades instead of keeping on telling different shit in a desperate bid for more votes.