Pheromones play a major roll in sexual selection. They kick start an initial attraction and in some cases serve to keep a couple together long enough to allow a more permanent bond to develop.
Over the last 20 years or so of being single again I must have read about 50 books on this topic and own at least 20. It is all very complex. What I have found is that in my life whenever I run into old girlfriends or a woman I had a crush on in the past (especially if she gives me a hug - translation - we each inhale the other's pheromones), I feel the identical feelings I felt even 50 year ago and I sense they do, too. This I find quite remarkable. Although attraction exists in each case, it is a bit different in each case. I cannot explain it but I feel it.
This article references the interesting sweaty T Shirt study mentioned above.
http://beheco.oxfordjournals.org/content/14/5/668.full