Why do stresses weaken or change the shape of something?

Answer 1

time for my materials science background to thrive!

stress doesnt neccesairly weaken an object.... depending on what you mean by stresses though this answer differs also for what type of material you are talking about.

Actually for some objects (solid crystalline/ others) adding stress will strengthen it to a point (check out a stress-strain chart).

But for physically weakening... repeated cycles of stress and strain will create a propegating crack which will eventually run to a critical point.

As for changing the shape, depending on how you mean this and for what type of object... it differs... in a crystalline object atoms can slide on slip planes (planes of orientation with least resitance).