system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How does a nose just spontaneously start bleeding? Where does the blood come from?

My friend gets super severe nosebleeds all the time, but I’m so confused as to where the the blood comes from so suddenly? What are these “floodgates” that open and why?
gpt
there are blood vessels in every soft tissue of human body 

inside the nose, the "skin" is very thin, and blood vessels are close to it. if their skin is extra thin, nosebleeds can happen easily. 

things that cause nosebleeds are dry air (which cracks the skin), or sudden increase in blood pressure (e.g. from getting excited over something). If these factors are present, then simply sniffling can be enough to start a nosebleed.