They really made The Grinch a sympathetic character...

I like it. We learn exactly why he hates Christmas and we're given an insight as to how it all came to be.
In the book and the 1966 cartoon (which are both excellent, I'm not putting them down in any way), depict the Grinch as just an evil guy who hates Christmas because it's noisy. Plus he thinks all there is to the holiday is getting gifts, but the Whos have the spirit regardless so the Grinch learns the true meaning of Christmas.

In the movie, both Grinch and the Whos learn the true meaning of Christmas. Here, the Whos are depicted as self-centered, materialistic idiots. Their mayor is just plain rotten and he played a big part in how the Grinch came to hate Christmas as well as the Whos.
As a tiny Grinchlette, he didn't necessarily have anything against Christmas at this point. He was just being mischievous, as was his nature. But when he met Martha May Who, he TRIED to be good, he TRIED to be generous, but the young mayor and his Who pals insulted him and made him feel terrible, so the Grinch went on a rampage and ran away to the mountains, and stayed there ever since.
I think the Grinch is depicted much like Ebenezer Scrooge, hating Christmas because so much crap happened to him on that day.
Not just when he was a kid, but look how they treated him at the Whobilation: tossed him around in a chair and force fed him pudding, fruitcake, fudge and egg nog. The way the Whos act and how they treat people, is any wonder the Grinch hates them?
So not only does the Grinch think Christmas is just about gifts and decorations, so do the Whos. As you could see, they put all that before family. But through the Grinch they learn the true meaning and all are happy, except their evil mayor. The Grinch learns too through what he did, so this time they both came out even.