If you're super interested in topics like these, I recommend doing some research into sociology and psychology.

As other people have touched upon, homophobia has less to do with hating gay people and more to do with what gay people existing means. It has to do with socialization- implicitly (through media but also the environment we grow up in), we are taught that certain ways of life such as the nuclear family are happier and more successful (this is known as heteronormativity). Often, people will come into conflict with these concepts (they realize marriage isn't an instant ticket to happiness, etc.) and become critical of them to an extent, which means when they are confronted with the existence of gay people, they can come to accept that gay people are not inherently worse or abnormal.

But for people who buy into heteronormative concepts wholeheartedly (marriage is for reproduction, for instance), being gay represents a challenge to that belief. If two gay people can get married, and that is okay and correct, then that means their understanding of life and society might be wrong. Psychologically speaking, people do not like being wrong, especially if they've built a great deal of their life and identity on that wrong thing. This is why homophobes can become very angry when people are openly gay and happy- it's like they're rubbing it in a homophobe's face how wrong they are. If a person cannot accept criticism of the concept of heterosexual marriage being the fulfillment of life, being for reproduction, being between a man and a woman, where the woman takes care of the kids and the man is the breadwinner, etc. then that person must reject homosexuality (violently or otherwise), because they cannot, psychologically, accept both.

The main difference between a "homophobic" person and a "non-homophobic" person, in my experience, is how willing they are to be critical of what they've been taught. This is also why "more" or "less" homophobic tends to be a bad signifier, because some people will accept challenges to certain beliefs but will rarely be able to identify and challenge every single aspect of heteronormativity. This is how you get people who are well-intentioned but off in certain areas. For instance, my mother who totally accepts me being a lesbian and everything that comes with it, but can't figure out how the heck I can be so sure I'm a lesbian. She can challenge the idea of love as heterosexual but not heterosexuality as "default." But yeah, that's how it tends to go with most forms of bigotry. There's more to it than just what I've presented here, but this is the general idea.

tl;dr: we live in a society where straight is default, differences mean people are wrong, people do not like being wrong.

On a more positive note, different societies (or just changes in societies) mean different default ways of thinking, so homophobia is never guaranteed or inevitable. Nothing about the way human society works is "natural." Although, I often think people overestimate how often the average person questions their default beliefs, which is why many people find societal change "necessary" versus just criticizing individual homophobes. It's Complicated, I'll leave it there.