Hi, just wondering, why does hardware slow down over time?

I've had my Dell pc for over 3 years now. Since then it has slowed down a lot and of course I am aware pc's are always faster when they are brand new. Why is this the case?

If I wanted to replace parts of my pc what parts should I replace to boost its performance? Motherboard? CPU? RAM?

Also, why do these parts become slower over time?

If anyone could answer these questions I would be truly grateful :D. Thank you! :)