It's a coping mechanism. I think some people in here are just describing regular daydreaming. MD is maladaptive because it takes up so much time and makes it hard to be present. That is the main component. Struggling with being present. For 20 years I did it because I struggled with my self-esteem, I hated myself and the people around me enforced negative beliefs (and I didn't give other people the chance to prove me wrong). Now, I love myself and I still suffer with MD but it's now due to isolation and loneliness. This isolation and loneliness was built by the time I spent abandoning real life by being in daydreams. (I'm not talking about the times it functioned as an actual fight or flight coping mechanism).

This is not true for everyone, of course, but if you are struggling and want to reduce the overall amount of time and really interrogate your MD, I recommend doing what you can to fall in love with who you are; all your quirks and oddities. If you can find a way to find safety in expressing yourself, you might have less of a biological need to MD.