Answer:
If the "..." is supposed to be a blank to fill, I think you could fill it with "tissue" or "cold".
"I have got a tissue because I sneeze a lot"
This works because it shows that you understand your frequent sneezes, and are preparing for the next one by having a tissue to sneeze into.
"I have got a cold because I sneeze a lot"
This also works because if you are sneezing frequently, it's probably because of some underlying medical condition, like a cold.
