People overspend for primarily a few reasons.
1. The urge to "get stuff". None of us can go anywhere without seeing things that would be good to have or get.
2. Lack of financial education and ordered thinking. People do not understand how to handle money well, and make serious mistakes without grasping they're mistakes at that moment.
3. Lack of self discipline. Many know what they're doing, but do it anyway, not taking future consequences seriously.
Of course, there are emotional disorders and mental disorders that cause people to act irrationally, and some include money. However, that's not the subject here. In reality, any one or combination of the above three, and possibly other factors, is generally why.