A "know-all" or in US a "know-it-all"
The word for someone who thinks they can do no wrong is "self-righteous." Self-righteous individuals believe their actions and beliefs are always morally justified, leading them to be judgmental and intolerant of differing perspectives. This mindset can hinder personal growth and damage relationships due to a lack of empathy and humility.
a snob maybe?
"Thinks" is the third-person form of "to think", which is a verb... so no, it is not a plural noun.
The term for someone who thinks of others is "considerate."
egotistic ego-centric
A person who thinks highly of oneself is conceited.
It is a word for a person who always thinks they are sick or might get sick.
Chauvinist-morbid patriot
A person who thinks they are better than others is often referred to as arrogant, conceited, or elitist.
WRONG
A person who is never happy can be called a malcontent.
Stubborn or uncompromising.