Monica Geller isn't the one who is in the hearts of many, here are some reasons for it:

1-she's a control freak

2-she wanted to waste Chandler's life savings on her wedding dream

3-it's either her way or no way

4-she dated Rachel's ex (which Rachel wasn't fine with it)

5-she was ready to sell Chandler down the river because she didn't want to "lose" to Rachel and Phoebe

6-when Chandler wanted to let the dog go, she sneaked it back in, when Chandler wanted to keep the duck and the chicken she said she had "allergies"