His positions on issues are not a problem for me. In fact, I'm totally with him in most of them. I've often spoken very highly of Warren, and how can a person love warren but detest Bernie if it's issues driven? They can't.
It's not as others have suggested and jealousy at how he can energize a segment of new voters, especially the young/millennials. That's something that I likewise love about him.
It's all about the large volume of over-the-top attacks that he's been delivering for years against the Democratic Party, the DNC, just about every long time Democratic Party leader that hasn't bowed down to kiss his ring (which is almost all of them).
Agree with it or not, even if it's fair or not, there's many who do blame him for putting Trump into place in the first place as a result of this rhetoric mentioned above. Something that definitely did not help this lingering bitterness was his staffing of people at the top of his campaign that actually were open, boisterous, and completely unapologetic Stein supporters.
Worse, many of us are seeing him doing the same thing this time around, and see his antics as helping usher in a 2nd 4 years of Trump.
I can categorically say that if his campaign was focused on attacking Trump (instead of the Democratic Party), not only would I be so much more open to support him, but he'd likely be on the top of my list of candidates to support. Hell, if he'd really change gears, stop the attacks, and get his campaign and most vocal surrogates to stop the attacks, I'd probably have him in my sig line now.