I have a slightly different take than the OP, albeit perhaps more abstract.
I always think of Boston fans as among the hardest on their own teams, at least this was my impression up until about ten years ago. Boston teams were the lovable losers, but losers for sure. I think it had something to do with the Red Sox, who kept getting close, then blowing it all.
The Celtics on the 80s were s rare bright spot, but then. Birds back went out, and Reggie and Len happened. And Karma seemed restored. We were losers again.
Then we had the Pats and Red Sox of the early 00s, and then the Celtics, and then the Bruins. And I noticed a big change. People in Boston started to expect their team to (gasp) win.
But there's now been a few years' drought, and I see Bostonians reverting back to old form. Reverting back to tradition. Namely, fear of expecting too much, bc it always ends poorly for us.
So, I don't see it as sad, but just the way things are in Boston. It's normal.
Is this referring to Boston fans? Believe it or not, most of us here aren't even from Boston! I, for one, don't care for any other Boston teams, just the Cs. But I do believe we are rough on our fans. You know the old saying... "hating the one you love most." Seems right.
But it is an interesting theory.
As a Boston fan, I can vouch for this. I don't really get this "rah rah" attitude Cs fans outside of the Boston sports circle have about the team. Its the same guys you hear talking about what an accomplishment is is to take the heat or lakers to 7 games as if that was some achievement to hang our hats on

.
Until these guys show some grit and determination to pull out the tough ones when it really counts, they're deserving of any pessimism and skepticism they receive. The 04 Sox had it, the 01,03,04 Pats had it, the 08 Cs looked strong throughout the regular season, and the Bruins won like four 7 game series on their way to getting it done.
What have these 13 Celtics done to show me anything? Not that I don't think we can beat the Knicks but I'm talking about winning it all. For me and other true Boston fans, anything else is just another wasted season.