How do I get -2 Reputation for some of my questions? They are sincere questions. And I don't have the reputation to tag this with reputation.
So, the short answer is: because someone thought they were unclear, not researched, or not useful, and downvoted them.
Stack Exchange is a community that works differently to regular forums, and has its own rules and guidelines. There are a lot of potential pitfalls for a new user, so I really can't recommend strongly enough that you skim through the help centre to learn how things work around here. A quick glance through some of the meta questions in the faq category might be helpful too.
Questions get upvoted & downvoted by readers, based on whether they think the question is a good one or not.
Hovering over the up/down buttons gives you a quick summary of the criteria:
This question shows research effort; it is useful and clear