Writing Poetry is an expression of thoughts, feelings, observations and reflection.
Whether its love, passion, nature, sorrows and sadness, hate. All can in fact inspire one to write and express. A reaction to sadness for instance, dependant on how you react can in fact be used to express that feeling in a poetic manner.
Instead of depression, one may express in a positive thoughts on how they have overcome such feelings, a love gain or lost can also be express in a poetic way. Therefore giving way to an inspiration that all that read the writing can either be inspired or puzzled