[A_Brandybuck]

In my opinion, there was more than only rejecting his 'doctrine'. Varda was not the only one, who went not with him and rejected his 'doctrine'. There were lots of other Ainur, who went with Manwë and haven't followed Melkor.
There must be another reason, that Melkor started to hate her. I mean, Tolkien mentioned only Varda in this case, consequently she must have done something, what the other Ainur have not done.

I see the reason in a special rivalry of the 'brothers in mind' Melkor and Manwë. Both are really powerful Ainu and plays some special parts in the thoughts of Eru. But both represents the opposite in the concepts, which they have. Evil versus Good.

It seems to me (I am only speculating), that both Ainur wanted Varda as spouse. But she rejected Melkor and became the spouse of Manwë. Considering the rivalry of Manwë and Melkor, this must be very hurtful for Melkor to see his chosen spouse going to his rival in mind.

That could be an explanation of the (from Tolkien) underlined hatred of Melkor against Varda. Refusing to being spouse of him and then becoming spouse of his rival can be a reason for very personal feelings and when you are assessed like Melkor, this will be manifested surely as hatred.