I just realized something . . .
Hobbes never calls Calvin "Calvin".
He calls Calvin's parents "your mom" and "your dad" (and then yells MOM when they get lost) but simply doesn't call Calvin anything at all.
What if this is him knowing he's actually a part of Calvin? He feels odd referring to Calvin by name because he is basically Calvin's conscience in tiger form.
This could also be why he agrees to go along with the things Calvin wants to do (most of the time), because he knows that eventually Calvin will stop talking to his imaginary friend and then he won't have Hobbes to brighten his day anymore.
Would this not be really depressing and touching at the same time?
If any of you can find a time when he does say "Calvin", let me know (I'm doing this mostly based on memory because I'm too lazy to page through the books even though I have them all)