Our reason for not having baby (we been married for 6 yrs n we both r 30) is that we dont need something else in our lif to keep us happy (or is most cases sad)..... seen too many couples struggling a lot with having...... 
We r happy the way we are... just want this to last forever.................... its not tht we dont like babies, we do..... we look after our nephews a lot of time.... but definately dont want baby for 24 X7 for next 18 (possibly more) years...... Life is as good as it gets for us...