Love is both rational and irrational. At the beginning of life, there were two main objectives. 1: To keep living 2: To make as many offspring as possible and keep them alive. Most things in life revolve around this. Being athletic attracts the opposite sex by saying "Look I can run faster than the guy next to me so he'll get eaten first" Having money means that you can look after your offspring better. Heck, even things such as being tanned show that you have the time to sit around doing nothing, showing that you're in a good position. Love for people you aren't likely to mate with is to survive longer. If your village is attacked you want to get your friends out so that they can get you out later. However, if your partner passes away and you spend the rest of your life in mourning then you aren't likely to fufill either requirement.