Also I find the following example from IRS. So if someone has a full time job (other than real estate related) and lend money, looks like he/she does not need to file 1098. Of course, he/she has to pay tax on every penny he/she receives as interest. Correct?
[Not in the lending business. If you receive mortgage
interest of $600 or more in the course of your trade or
business, you are subject to the requirement to file Form
1098, even if you are not in the business of lending
money. For example, if you are a real estate developer
and you provide financing to an individual to buy a home
in your subdivision, and that home is security for the
financing, you are subject to this reporting requirement.
However, if you are a physician not engaged in any other
business and you lend money to an individual to buy your
home, you are not subject to this reporting requirement
because you did not receive the interest in the course of
your trade or business as a physician.]-from IRS