system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do muscles stiffen and lose flexibility? And why does stretching sometimes feel good and sometimes hurt?
gpt
Lots and lots of reasons.  But ELI5.  Muscles get stiff because they get used to being short and all the fibres get tighter and closer together.  It can also be because of literal knots in the muscle.  Imagine you cut a piece of string in half, to make it whole you have to tie a knot in it.  The string is shorter but it’s whole.  These are knots and there can be thousands.  Thanks to healing and massage those cuts can be healed to normal. 

Pain when stretching is normally due to excessive tearing.  It’s your body screaming at you to stop.  It feels good because of other reasons that I’m not clear on.