Some of us just have inner competitive spirit which isn't backed up by any real physical talent or with motivation/time/energy to train.
Still, if going a little fast is fun, then by logic going much faster will be much more fun.

Doing short sprints helps me work on my strength/speed.
It's fun chasing people to see if you can pass them. It's more fun than structured intervals just going against the clock.
It helps when fatigue has set in and motivation lags.

I like to pass people pretty quickly and get back on a good line. This is especially true on MUPs where I'm crossing over the center line to pass. So yes, I do accellerate to pass then slow down once I have good clearance.

Sometimes I just want to get away from a rider ahead who is acting squirrely / safety hazard / creepiness.

Sometimes I just want to get some momentum to ride into a hard uphill when I have a lot of commuter stuff on my bike. Did I just pass somebody? I didn't notice and I don't care.

Sometimes I am just passing somebody because I have a broad range of speeds on my commute routes (5mph uphill, 35mph downhill, 17mph flats) and I am in the terrain where I go relatively fast.

Sometimes I just get mood-altered and can't control the urges to chase. It's much more fun if they will play too. The time goes by faster playing a friendly game of cat and mouse. The game is over if somebody breaks traffic rules or rides unsafely.

My rides are about my personal goals. I can't control what other people do or usually know what their ride intent is, what they brought to their ride that day. Most likely if I do know, I am quite humbled.