Why does iPhone automatically delete calendar events from months ago?

And how do I make it stop? I use my iPhone calendar a lot and sometimes I rely on it by scrolling to previous months to see when I went for certain appts but it automatically deletes last events without asking? I know I can make special events like bdays repeat weekly or yearly but some events don't require that function, I just want to know when I went to my appts for example, what day and time it was my dentist appt.
Can anyone help?

Change the setting.

Settings > Mail, Contacts, Calendars

Scroll down the screen until you reach the section on Calendar and change the "Sync" setting to something longer.