Some people need to have their ego stroked elsewhere when they experience relationship problems.
It gives them reassurance because getting attention and possibly the offer of another relationship from someone else gain any insights from their relationships.
Whoever they overlapped with may feel flattered now and be enjoying the honeymoon glow but they don’t realise that your average overlapper and cheater is demonstrating that when faced with problems and conflict, they don’t do problem solving.
Eventually they may find themselves being overlapped too.
If the possibility of breaking up hasn’t been discussed, at least one of you is thinking about it.
The overlapper may justify their actions by claiming that the relationship was “practically over” or claiming that they told you they needed space.
Let’s be real: some people use knowledge of a possible imminent breakup to be ‘open’ to new possibilities.