Many of the things that we as humans do are irrational. Insanity is different, because we are 'aware' that what we do is irrational but enjoy the activity and so continue.
For us to enjoy something, it does not have to have a purpose. If we find something gratifying, we can become addicted. Many of our hidden emotions are played out through irrational activities. Driving too fast, killing spiders, dreaming, religious belief, and arguing on an Internet forum, to name but a few.