I vote ‘False’. Along with the materialist assumption, the rationalist assumption is at the core of many modern errors (e.g. Marxism, which assumes human needs are essentially material in nature and can thus be satisfied and controlled through rational means.
As any one with a modicum of life experience and honesty will admit, this is a bogus assumption — everyone, at some level and at some time, is motivated by things which go against any subjective measure of their self-interest (romantic love or erotic attraction is a perfect example, or anything else touching on aesthetics for that matter). For instance, I’m of the opinion that the genius of capitalism is that it’s essentially agnostic when it comes to motivations — if a person wants to spend their time and treasure purchasing trinkets or chasing loose women (or men) and they have a useful skill to offer, a capitalistic system allows them to labor and contribute (As opposed to Marxism, which for all its materialism has a very moralistic view towards motivation — status is a very strong but also less than rational motivation, and one that Marxism is intractably opposed to).
As Augustine said, Magnum mysterium mihi (I am a great mystery to myself).