Not everyone that becomes a landlord knows quite what they're getting themselves into. Some people become 'accidental landlords' when they find the market isn't right to sell their house as quickly as they'd like or they hope to move back in someday. They didn't expect to become a landlord, but to pay the mortgage, something needed to be done. Now they have a tenant in the home that they once lived in and there are expectations that they never even thought of.
Even those that choose to be landlords do not always do the research that they should do before undertaking their new profession (and yes, this is a profession, not a hobby). They may have thought that all they needed to do was buy the property, sit back, and watch the money flow in. That would be nice, but unless you're willing to hire and pay for a property manager, there's not a lot of sitting around that goes on with being a landlord. You'll need to get the property into shape, market it, go through the tenant screening processes with the interested parties, and choose a new tenant (all while abiding by the fair housing guidelines). Once the new tenant is chosen you will sign the lease with them, possibly do a walk through the property with them before they actually move in, and then you'll be the one that they turn to when they have maintenance issues, any questions or complaints about the property or other tenants that are renting from you, etc etc. If they don't pay on time you're the one that will need to follow up with them and, if they refuse to pay, start the eviction process.
Those are just a few things that a new landlord will need to know when embarking on this often fast-paced journey. BiggerPockets has an article out about the 5 top qualities a person needs if they're going to be a landlord. Check that out here
to see if you match up.