We had problems with our first but our second has just slotted right in, DS1 has been excellent with DS2, DH and I are as happy as ever.
I do put it down to lack of sleep, our first was a terrible sleeper, would never just drop off to sleep on his own, he was BF half an hour on half an hour off for the first two weeks, I was just drained and exhausted and had a lot to deal with after the labour, which I felt I had failed miserably at, and then finding myself fat (going from a size 8 to an 18 wasn't nice!) and full of stretch marks and saggy bellies, YUK! I was certainly more prepared for the impact of the pregnancy and birth on my body second time round.
And I guess age as well I was 18 when I had my first and although he was planned and we were married and happy, I did find it hard, second time I was/am 23 and more able to deal with the emotional side.