[The Might]

Clearly, there was a change in him

As noted in the Ainulindale, when the Ainur sang to him in the beginning, Eru was glad
But afterwards they all spent some time in which they "sang only each alone, or but few together."
It was in this time that Melkor had changed and started seeking the Flame Imperishable, as he wanted to create things of his own.
I don't think a friend would have helped...he wanted power
And since it was hard to rule over his own kind with Eru around, he wanted to create his own things over which he could rule
I think he wanted to be just like Eru, and have his own place where he could rule everything. And since this was not possible in Ea, he would have destroyed everything had he succeeded. I think it's hard to imagine how spirits think, or what needs they have...I'm not sure about this matter.