Dentures can break for a number of reasons. In fact, most dental professionals recommend that you change your dentures around every seven years or so. This is because as you grow older, your gums will keep receding therefore causing your dentures to become looser. This makes them more uncomfortable to wear and more likely to break as your gums are no longer giving them sufficient support.
The other main reason that dentures break is because they simply are not as strong as natural teeth. They are likely to suffer from daily wear and tear through talking and chewing, this makes the dentures weaker and with the majority of wear usually on the posterior teeth it increases the pressure in other areas, therefore causing them to break.