Evolution depends on procreation of offspring that live to successfully procreate themselves. When a woman becomes pregnant she is vulnerable, and, in our past, would not be able to look after and feed herself. When she gives birth this remains true, and the early years of the infant have a high relative mortality rate (by this I mean that if a child survives the first couple of years, there is a high probability it will make it to adulthood).
This means that it is an evolutionarily fit strategy for men and women to couple off for at least a few years – to ensure their genes are passed on successfully.
When choosing a mate, you want to select the best mate that will also be happy to have you, and for that selection period to not be too long. As an interesting aside, game theorists can prove the best strategy for a game like this is to evaluate the potential mates you meet for 36.8% of your acceptable time period (1/e) and then mate with the next one that comes along that is better than the best option of the evaluation period. For example, if you wish to settle down between the ages of 18 and 32 then 37% of that window is 5 years, so you should settle for the first person you are presented with who is better than anyone you've met up to that point at the age of 18 + 5 = 23 years old.
What if a better option is presented to a man in the period shortly after he gets his woman pregnant? Would it not be in his interest to switch? Yes, but if this happened at a high frequency in a population then it would not be a good strategy on average. A woman needs to believe that her man will stick around and so men do not tend to 'decide' to be with their mate for rational reasons that could be trumped by a better woman round the corner, but they fall in love instead. Love acts a bit like the doomsday device explained in part 1; it says to a potential mate, “I am with you for irrational reasons, and so there is no reason why I will leave you.” The feeling, to be trustworthy to the mate, must be accompanied by difficult to fake physiological symptoms.
It is possible to take a blood test to see if you are in love. The telltale chemical/hormonal signature lasts approximately 30 months. This is, predictably, roughly the length of time between conception and the child exiting the high-risk early years.
When a man has sex he is less committed to caring for the woman and child, whereas if the woman gets pregnant, she is committed to becoming more vulnerable and expending a lot of resources to have a child. Therefore it makes sense that sex will lead to feelings of love more easily for women than men.