Most renters will only place a complaint when something is truly wrong. For example, a little while ago I had to list my first complaint to my landlord because of a small hole in a pipe behind our sink. The water had been leaking out of the pipe and had begun to cause water damage in the apartment underneath. This was the first thing I have mentioned to my landlord since we moved in last year, as all other issues I have simply dealt with myself. Naturally, however, this was something that needed a professional touch and therefore needed to have the landlord's approval. Other tenants may not react to issues that occur in the same way.
Some tenants do have quite a habit for complaining about every small thing. This may be for any number of reasons. Depending on your tenant's age or physical mobility, they may not be able to handle the small day to day issues on their own. If they have no one else in their lives or in the area, then helping them with these things may very well fall to you. Hopefully you will be made aware of this before they move in, however things can certainly happen after someone moves in which necessitates extra help. While we always hope that people will remain healthy and happy, this is not always the case.
Handling complaints from your renters can be a rather exhausting activity depending upon your specific situation. The one thing that is most important to remember is that you are normally the easiest person for your tenant to contact when something goes wrong in the apartment. You are the person who needs to know what is going on, after all.