Research the Gap Concept a bit for your answer. There is a section in Tournament Poker for Advanced Players by Sklansky which discusses this.
Basically, it takes a stronger hand in your position to call an opponent who opened before you in earlier position.
http://www.pokerlistings.com/tournam...he-gap-concept
The term "gap concept" was first introduced by the excellent poker writer David Sklansky (see Editor's Pick for book reviews). This concept means that you will usually need a better hand to play against someone who has opened the betting than what you need to open with yourself. The difference between the hand needed to call an opening bet with and the hand needed to open with is called the gap. Depending on whether your opponents are playing tight or loose, the width of the gap changes. The tighter they play the bigger the gap, and the looser they play the smaller the gap. This means raising when first in with a lot of hands you would never call a raise with normally. If you are sitting in late position playing Hold'em with a decent stack and no one has entered the pot, it would be correct to raise with hands as weak as 22, A-x and K-9s. Of course, if a very aggressive player is sitting in the blinds and he plays back a lot you will have to be more selective.