The Cynics

Cynicism is a school of thought of ancient Greek philosophy as practiced by the Cynics.As reasoning creatures, people can gain happiness by rigorous training and by living in a way which is natural for themselves, rejecting all conventional desires for wealth, power, sex, and fame. Instead, they were to lead a simple life free from all possessions.(1) __

A Cynic has no property and rejects all conventional values of money, fame, power and reputation. Cynicism denotes a way of life as well as its corresponding mind set. It is through their practices, the selves and lives that they cultivated, that we come to know the particular Cynic ēthos.
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 Antisthenes (c. 445–365 BC)was an older contemporary of Plato and a pupil of Socrates. About 25 years his junior, Antisthenes was one of the most important of Socrates' disciples. Although later classical authors had little doubt about labelling him as the founder of Cynicism, his philosophical views seem to be more complex than the later simplicities of pure Cynicism. In the list of works ascribed to Antisthenes by Diogenes Laërtius, writings on language, dialogue and literature far outnumber those on ethics or politics, although they may reflect how his philosophical interests changed with time. It is certainly true that Antisthenes preached a life of poverty (1)
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Diogenes did however adopt Antisthenes' teachings and the ascetic way of life, pursuing a life of self-sufficiency (autarkeia), austerity (askēsis), and shamelessness (anaideia). There are many anecdotes about his extreme asceticism (sleeping in a tub), his shameless behaviour (eating raw meat), and his criticism of conventional society ("bad people obey their lusts as servants obey their masters") (1)
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Crates (365 – c. 285 BC) of Thebes was a Cynic philosopher. Crates gave away his money to live a life of poverty on the streets of Athens. He married Hipparchia of Maroneia who lived in the same manner that he did. Respected by the people of Athens, he is remembered for being the teacher of Zeno of Citium, the founder of Stoicism. Various fragments of Crates' teachings survive, including his description of the ideal Cynic state.(2)

__As Diogenes ’ reappraisal of shame suggests, the Cynics are not relativists. Nature replaces convention as the standard for judgment. The Cynics believe that it is through nature that one can live well and not through conventional means such as etiquette or religion. One reads that Diogenes of Sinope “would rebuke men in general with regard to their prayers, declaring that they asked for things which seemed to them to be good, not for such as are truly good” (Diogenes Laertius, Lives of Eminent Philosophers, Book 6, Chapter 43). This captures the crux of the Cynic notion of living in accord with nature and contrary to convention. Praying for wealth, fame, or any of the other trappings convention leads one to believe are good is a mistaken enterprise. Life, as given by nature, is full of hints as to how to live it best; but humans go astray, ashamed by petty things and striving after objects, which are unimportant. Consequently, their freedom is hindered by convention. (3)
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Further reading : 
Tips for changing your cynical view of yourself, others and the world: 
1. https://paulocoelhoblog.com/2017/04/20/how-you-can-defeat-cynicism/
2. https://www.aconsciousrethink.com/8517/stop-being-cynical/
3. Dealing with Cynical People: https://outsmartyourbrain.com/5-steps-dealing-negative-cynical-people/
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References:

1. https://en.wikipedia.org/wiki/Cynicism_(philosophy)
2. https://en.wikipedia.org/wiki/Crates_of_Thebes
3. https://www.iep.utm.edu/cynics/