@sylvie There can be a lot more to it than just "not wanting to be alone".
New relationships cause a huge surge in dopamine production.
For anyone who has issues with dopamine regulation (like ADHD, etc.), they're apt to discover early that they "just feel better" when they're dating someone. Even if they don't fully realize, they can wind up using relationships as a form of self-medication.
Where it gets ugly is that those levels of dopamine production from the relationship drop off a little after 2-4 months, and then a lot around the 2-4 year mark.
That's hard enough to deal with if you know that it's coming, but a lot of people have no idea and assume that their relationship is terribly broken and their world is falling apart when they start to return to their body's normal baseline levels of dopamine.
Hitting that wall can lead to people making poor decisions like cheating or abandoning stable relationships to chase after something "more exciting".