My first reaction is that it is the life motto of libertarians. A deep rejection of paternalism. Like many libertarian economists say: 'no one spends his money as wisely as their own'. People have an incentive to take care of themselves and their own property. Others disagree like many conservatives. Roger Scruton, for example, says that education is a way of society to make people free -- and approaches this issue from paternalism. Progressives often agree with Scruton on this point, they highly value the use of media, expectations and education to change people to their ideals, another paternalistic approach.

It is ultimately the battle between paternalism and rebelism.

It's better to think that one knows what's best for them, even if they don't, than to think that someone else knows what's best for them, as the latter is a dangerous way of relinquishing control over one's life to somebody else and ultimately denying responsibility for one's own actions.

I don't think it entails that. You can acknowledge someone doesn't know what is best for him without dictating his life actions; it can allow for healthy, constructive discussion. If a person's convinced taking cocaine every few days is okay despite you knowing he has an addictive personality, I think it helps to assume he doesn't know and bring that up to him, challenge the belief in a non threatening or condescending way so you can have a constructive discussion about his drug use and his beliefs regarding his drug use. Maybe it can get him to reflect a bit and entertain other perspectives.

And there are cases where relinquishing control in certain decision domains is a good thing. Relinquishing your control over health related decisions to a field-expert in a hospital is an example of that. And the caregiver isn't actually suppose to provide treatment according to his/her own conception of what's ideal, they take your values or interests (desire to live; desire to maximize life quality; whatever it may be) into account in the treatment process- they just identify the most research informed way to go about getting there.