Hi,
The other answer you received before this one is very good.
No, it's not always to please someone.
If you would like to read a little more, here is a link:
http://www.chacocanyon.com/pointlookout/031105.shtml
The link compares dogs' feelings to similar human feelings, and is some good reading!
I agree with the dog tail wagging as meaning excitement. Trick is, come to know and how to read the other signs given by the other answer, and some of those are in the link also.
I do wish you the best.