Katy Perry & Russell Brand Divorce -- The Die Was Cast Early On

Sources close to Katy Perry and Russell Brand tell TMZ ... the couple was infatuated with each other when they tied the knot, but the honeymoon ended very quickly.

According to our sources, Katy and Russell were initially head over heels for each other – each fascinated to be with someone who was an extreme opposite.

But we're told the two began a regime of constant bickering after they got married -- often in front of people at clubs and awards shows. They're both very stubborn and neither would easily back down.

Our sources tell us ... because of the friction and their schedules, they never spent long periods of time with each other.

One big bone of contention ... Russell was much more of a homebody than Katy. Katy loved going out and partying, while Russell (who is sober) didn't enjoy the scene at all. We're told he would often insist on leaving places early and asking her to come with ... which would trigger an argument.

But we're told ... as the dust settles, neither hates the other -- far from it. There's no bad blood between them, and neither of them believes the other cheated.