Life without kids was free and fun
Life with kids means less money due to childcare, giving up work, taking 4 on holiday, additional costs, probably a bigger house etc. etc. The dad generally works hard, keeps his head down provides for the family, gives up their interests to spend time with family or working on the house, painting, gardening, washing cars etc. Finally wakes up one day and thinks 'What about me?' I work 48 weeks a year, long hours, don't do what I want, bored, dejected and I am suddenly getting old. A midlife crisis is the explosion from here, doing an extreme change and doing something mad.
I am making sure I don't give up things I enjoy, I spend time playing PC games, listening to music, photography, sailing and karting to make sure I am happy and I am happier to be around and won't have a midlife crissis. It is important to have downtime so you don't go mad/turn into a working zombie.