Why do some people monkey-branch in relationships?

Why do some people "monkey-branch" from relationship to relationship, i. e., they are always in a relationship without taking a break, are never single, and often their new relationship overlaps with their previous one?

Cheating is often involved too, as monkey-branchers often start entertaining the next person emotionally or sexually while still in a relationship with someone else. Then, when they've secured the new person, they break up with the old one and "swing" to the new relationship. Like a monkey securing a new branch to swing to.

What's your view on monkey-branchers?

Why do some people monkey-branch in relationships?
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