It's incredible how nice Charlie is to Alan
For casual viewers, Charlie may seem like an *beep* in his treatment of Alan. He constantly spouts insults, he sometimes screws up relationships for him (especially in the beginning of the show) and sometimes starts a feud over nothing.
But then again, he lets his brother live with him for nearly free (he has encouraged him plenty of times to get out of the house, but even then mostly has accepted the situation is how it is), he helps him out even when he find him the most irritating, he lets his son stay there too and he even lets him sleep with women at his house.
If anything, Alan is never grateful enough. He constantly complains about Charlie's order of things, that he brings women to the house all the time (It's his own goddamn house, for Christ's sake), he never pays the rent, bothers Charlie about the stupidest of things, and worst of all, takes it completely for granted. Charlie helped his brother out because he really was in trouble and he actually cares for him. But Alan has then continued to stay just because he "likes it" there. He even outright admitted it without shame in A Bottle Of Wine And Jackhammer (which was a very funny episode, but the reason for Alan staying was my one flaw with it).
Funny, it started out with Alan being way nicer than Charlie. But after season 2, it became much more evident Charlie himself actually is the nice one.