There was a better article somewhere, but first google result on the topic should suffice: http://www.dailymail.co.uk/femail/article-2284776/Can-parent...

As stated by Kahneman happiness (pain) level is felt in retrospect as an average of the most happy (painful) moment and happiness (pain) level of the last moment. If you have kids you have an easy happiness peak (say their first smile) and are likely to be less lonely in your old days.