What causes humans to have different sleeping types rather than just all having the same type of sleep?
Thalamus activity. When you're in NREM (non-rapid eye movement) sleep the thalamus creates bursts of sigma waves (so called "Sleep spindles"). These sigma waves burst around in the brain, transmitting from neuron to neuron, and are speculated to have a function in helping you consolidate and make sense of memories.
The more sigma bursts you have, the heavier you sleep.
P.S: While I haven't seen research on it I'm also guessing that your attitude changes how deep you sleep. At least I tend to sleep extremely deeply if I'm a place where I feel safe, even if the noise levels are pretty high, but if I sleep in a place where I can't lock the door or in a tent or if someone else is in the room...much lighter sleeper.
Aside from a few chemical balances, trauma can shape people into waking up towards certain triggers.
For some people, it's the sound of the garbage truck in the morning. Others, garage doors. For me, it's any physical/audio sensation that doesn't feel like it's from myself because of overlap.
It makes sharing a bed a very difficult if not impossible task if I need to actually sleep. Quieter, darker, secure, and familiar are the best bets on qualities for minimizing my tendencies where I can even wake myself up, and it's pretty similar with other light sleepers i've encountered.
Source: Experience and other testimonies.