Chances of getting 1 pair beaten by 2 pairs

[Thirdman]

Hi,

I'm finding it frustrating when I think I have the nuts with good pairs such as As, when it gets to the showdown I often find I'm beaten by someone who has 2 pair with a weaker high card than me and a weak second card e.g. I have pair of aces getting beaten by kings and twos, the two coming down on the river. Can anyone tell me what are the changes of getting 1 pair beaten by 2 pair and is a top pair actually a good result or should I play pairs less aggressively?

Cheers