My iphone cant remember networks! Everytime i come home i have to retype in my wifi password! Please help!

My actual wifi works fine but whenever i disconnect from my wifi for any reason (leaving the house, restarting my router, etc) i have to manually reconnect to the wifi in the settings and re enter the password every time. It refuses to remember any networks. And my cousin came over with his phone and he hasn't been iver for a long time. He still re-connected automaticly! So its certainly the phone! Please help!