I think the whole "Joining the darkside" thing
and his quick decision to do so makes sense.
He's basically been groomed by palpatine for over
a decade.
He knows palpatine and thinks he isn't a bad dude,
on top of that palpatine does a lot to appeal to Anakin emotionally
and they have a kind of father/son relationship.
He plays on Anakins fear of loss which I also think
made the decision pretty easy for Anakin.
I think their relationship isn't blatant but it's definitely
stated specifically by Obi-Wan

he says "be careful of your friend...."
or something to that effect, so of course their relationship didn't
just come out of nowhere.
Also the fight between Anakin and Obi Wan
