I'm just a bear, but this is what I have observed at various campgrounds.
-biggest reason like others have said is due to side effects. Weight gain, lack of sexual interest, perceived negative changes in personality, tiredness, feeling flat, etc. Imagine the secondary side effect of being in a relationship and feeling like you arent keeping your partner happy if you have a nonexistent sex drive. That could be external pressure or self imposed pressure on your self
-individual feels their issues have stabilized and feel they no longer need medication. Keep in mind with some meds it can take weeks to stop feeling the effects once you stop use, or to feel effects when you start. This can lead a person to think that they are doing fine off meds.
-outside of side effects already covered, the individual may feel being on the meds is negatively impacting their life. For example if you are a creative type like a comedian or musician you may feel you need your depression or mania for material or because you believe it makes you better are your craft in some way. Lot of humans with mh feel their ailment is a core element of who they are person.
-health and stigma. Look at how this thread will unfold. Lots of stigma about medication and people on medication. In addition lots of people just dont want to put a bunch of stuff into their body.
-consequence of mh issues itself. Individuals with schizophrenia often cant tell fiction from reality. They might not be able to tell you what planet they are on or think everyone is out to get them. How hard do you think it must be for this sort of person to stay on top of a daily medication regiment? Hell healthy functioning humans often dont stay on top of their prescriptions ie birth control, pain pill dosages, diabetes, etc. However according to the owl there are medical advances and a movement towards monthly shots for individuals for schizophrenia which would greatly increase medication compliance for many reasons.
-mh symptoms can be fun. Mania often feels great to the people who experience it.
As for me? I can assure you I'm getting my honey daily