Whether we wish to admit or it, laziness, inertia, and depression are somewhat interrelated. A mentally healthy person wants to accomplish his/her goals and to make some type of contribution to his/her society and culture. H/she is proactive in terms of wanting to be engaged and involved in his/her immediate and outer environment. A mentally healthy person is independent and is loathe to depend upon anyone; h/she desires to be self-sufficient.
A person who is lazy and inert is not normal. H/she has some type of mental illness which causes him/her not to want to be engaged in life. H/she has some type of self-esteem issues. H/she probably have a very low sense of self. H/she feels that h/she is insignificant in some way. Because of this feeling of low self worth, h/she believes that whatever h/she does really matters for naught and feels that it is utterly futilitarian to make even the slightest effort.
Since h/she believes that h/she is insignificant, h/she adopts a negative consciousness, mindset, and/or psychology which eventually becomes a self-fulfilling prophecy. H/she stops taking care of his/her appearance, starts to become progressively worse in terms of attitude and outlook, and/or h/she even stops interacting with his/her immediate environment and the outer society.