The following two links (really just one article in two parts) might help you understand some of the different views people hold on this topic and why they do (though they're only examined in brief). It's not an easy thing, and there has been disagreement for ages as to what the "answer" truly is.
http://clashingculture.wordpress.com...c-part-1-of-2/
http://clashingculture.wordpress.com...c-part-2-of-2/
You can find a similar thing here, only in
10-part YouTube series form.