Ok, so I completely support universal healthcare. However, it still is true that you are paying for "someone else's healthcare". How?

Let's assume that there's a flat tax percentage - 30% for all. (Actually most developed countries have progressive tax systems, but let's ignore that for now). The more your income, the more tax you pay. Therefore, some people pay more tax than others. This means, that some people contribute more to fund the healthcare system compared to others.

Some people have pre-existing conditions. Some people may just be unhealthy due to bad lifestyle choices. I might be incredibly fit. The probability of me falling sick would be very less. If there were a multi payer healthcare system, then perhaps I might not need to spend much money on healthcare. A universal single payer system might be forcing me to pay more for others' healthcare. Therefore, saying that I'm paying for someone else's healthcare isn't inaccurate.

That being said, healthcare is a human right. Every human, regardless of financial status deserves timely access to good healthcare. That's why I support it.