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This is an existing answer of mine; which I believe is on-point.

https://writing.codidact.com/questions/36209#answer-36211

Basically: Falling in love is generally a combination of sexual attraction, complementarity, and commonality.

Friendship (or platonic love) is formed by the last two, minus sexual attraction.

Commonality means you have things you enjoy in common; the reason this brings you together is synergy. It is more fun to do things you like with other people that like them; and if we both like, say, the same music, by sharing our finds we get more music we like than if we are alone.

Or Commonality can be a similar sense of humor; I say things you find humorous, and vice versa. For a lot of people a failure on that commonality is a deal breaker.

Complementarity is when I am weak in an area I wish I was not, but you are good at it, and vice versa; I am good at something you are not great at (and wish you were). This can also be synergistic; together we can be better at life than the sum of what either of us is alone. We make good partners.

Or some women like a man that can defend them; that is complementarity: They do not feel good at defense, and feel safer being with somebody that is good at it.

The complementarity has to be in a good way. I *wish* I was good at cooking, but I'm not, and I appreciate it when somebody can. It can't be opposites like one is a racist and the other despises racism.

You devise traits for the two characters that they can eventually discover as commonalities and complementarities.

For Romantic Love, sexual attraction is pretty much required; but it doesn't have to be immediate. When people are thrust together (like in a work situation, or some other "cauldron" that forces them together), they may not initially be that attracted to each other, but they can get accustomed to each other as people over some weeks or months, and develop a physical attraction. For that kind of story, you'd want some strong commonalities and complementing skill sets. A common compelling purpose (for example running from the mob, or escaping being stranded in a jungle) can also help a relationship develop into love.