Why does my iPhone keep freezing?

Sometimes my iPhone freezes and I can't do anything why

Too many apps
Old (maybe)
Always dropped
Too many open apps

Please check this one.

Go to Settings > General > About and look at the Capacity and Available numbers. If your memory is almost full you might try moving some data off of the phone.

if you are using iphone5 then you better have a look at this link:

https://discussions.apple.com/...5&tstart=0