They are in a situation they now cannot realistically change and that they are 'supposed' to enjoy. There is a lot of pressure to convince themselves they are now happier 'overall', even if actually each of their days is now less happy.

The fact is, you will encounter plenty of misery in life regardless. You should not choose based on what makes you happy, but arguably based on what kinds of misery you are more suited to - the misery of having children or the misery of not having children.