For me it was like this:
It starts out as just feeling incredibly happy. Just being able to talk to the person feels like the best thing in the entire world. You feel like nothing can stop you, like you can take on the entire world.
Once you get to know each other better and start getting closer, you physically desire the other person. You want to constantly be near them and "do" stuff.
At a certain point the initial infatuation dies down and turns into a more real love. You feel safe with them around, like you can trust them with anything and everything. Their presence can lift your bad mood and thinking about them make you smile. You want to do anything for them, help them up when they're down, make them laugh when they cry.
As for writing advice, I wager it will be pretty difficult to write about something as complex as love if you've never experienced it yourself. Though, while you might not have been in a relationship before, there are probably some relatable experiences in your life that you can draw from to write out a romance story.
