People tend to be quite different in the things they think about. Some people are always thinking about the future, and the good and bad things that might happen to them. Other people have their mind mainly in the present without much thought for the past or future. Other people have an orientation more towards the past.
It's normal to think about people and experiences that shaped our lives and led us to our present selves, our past serves as a reminder of where we've been, how far we've come and what we need going forward. whats wrong about reminiscing on the past is living in it, letting it take over the present and the future. It hinders you from making progress aside from that its bad to your health it kills you slowly.
Let me quickly give a few disadvantages.
You miss out on other opportunities
it ruins relationship
You waste your life.
At the end of it all you ask yourself, "was it worth it"?
The answer is NO