Psychology is the science of behavior and mental processes. In other words, psychology enables us to understand why people think, feel, and act in particular ways. Multiple factors influence personal development. Your genetic make-up, sociocultural factors, self-cognitions, and affect incessantly influence one another and thus the person you see before you in the mirror day to day.
Psychology is considered to be a "soft-science," whereas Chemistry, Physiology, Pysics, etc, are classified as "hard science." This distinction is beginning to lose significance in current theoretical studies. A hard science is an old science and has many solidified theories that can indefinately be proven mathematically. For example, Einstiens E=MC2 is proven by math. A soft science, such as psychology, is a younger science that has theoretical propostions, but cannot be proven without eliminating all possibility of error. We may say ,for example, that one thing leads to another, (serotonin deficiences lead to depression), but that may be only one of many factors infuencing depression. Such factors, including personailty dispositions, socioeconomic status, familial upbringing etc. have been found to concurrently influence depression. However, we have NO MATHMETICAL FORMULA that will conclusivly find who will and will not develop depression. Further, EVEN if we did have such a formula...we have no way of saying with absolute certainty that THESE are the ONLY factors....there is always the possibility that other factors (presently unknown) may also effect the development of depression or any other topic in psychology..