There is nothing wrong with watching show for Sam and Dean, for Sam and character B, or Dean and character C (or, heavens forbid, characters B and C). Unfortunately, some people who watch for Sam and Dean don't get this. So they have to keep hammering over and over how show can only follow two characters, how you're in the wrong if you follow other characters other than those, how you CAN'T complain if you feel character B or C was mischaracterized or mistreated, how character B or C are nothing but worthless fanservice, how character B or C are intruding, and so on and so on. THIS is assholish. Not preferring whatever characters you prefer.

There's countless ways to display a preference for a character, without the childish need to bash other characters and their fans.