Why the word “gay” change its meaning?

In not start of 70s or 80s it is not the meaning of this day. It means love and care-free while it is not yet 70s so why someone changes its meaning?
Update: Why they just make another word instead of mocking them with some persons name “Gay”?!
Update 2: What if do you heard the persons name “Dick”? It sounds someone’s mocking and add to dictionary. What a awful linguistic person.
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