My favorite interpretation is that Hutch's meanness is just a part of their well-honed relationship dynamic: slightly overdoing the basic roles they've drifted into, to amuse themselves and to let out some steam. (I've seen that happen between male friends in real life - and I admit I've been there myself sometimes, saying awful stuff just to cultivate some stupid inside joke or other, not really meaning any of it personally. But I suppose it's a brand of humor/bonding that comes more naturally to men.)

Also, considering what a gentle and sensitive person Hutch usually is, the way he picks at Starsky in particular must be his way of telling him "I love it that we're so close that I can do this to you and trust that you know I don't really mean it and read between the lines that I'm just saying 'I love you'." It's either that or he'd be all over Starsky, physically (which is what Starsky does to Hutch).

And it's not like Starsky doesn't gripe about Hutch's oddities all the time (health fads, so-called car and the mess in the back seat, opinion on Christmas, disastrous taste in women, blondness...) but maybe most of us don't really mind that because it's so easy to think Hutch's choices - surely written for laughs back in the day - beg for comments like that. And Starsky has less of a cold poker face when he says those things - his teasing is much warmer! It's almost like Hutch has copied this art of friendly sniping and practical joking from Starsky and is still doing it slightly wrong.

Uhhuh, got carried away there... :) Btw, are you on the S/H group on FB? It seems like the liveliest fandom haunt at the moment, and there are other new/newish fans happily squeeing over the show there. https://www.facebook.com/groups/SHbromance/