I'm going to state the obvious here and state what happens in rl.
When one is with another, a main couple of reasons are that they are fun and make them happy...So, in basic format, if you write about two characters I would put that into account. The two people need to have fun together and be happy, whether it's as simple as doing something they don't normally do (One might be from a different place and more active as the other may not be.) And happiness to be with another, normally could be something simple as well, most of the time it could be something that makes one feel special.
Examples of my romantic relationships: Mahnu and Richard, Richard helps Mahnu forget about her sufferings and he brings on a whole new experience that is much better then what she's used to. On the other hand, Mahnu treats Richard like a man, not a King, and he had to work to get her attention, in which made him fall for her more because he's never done that before. Richard makes Mahnu expand her boundaries as he treats her different to what everyone else had treated her. (There's more but this is the basic principals of their relationship.)
For Kael and Malory, Kael grew a firm protectiveness and possessiveness over Malory, before he even realized that Malory was a female. She ended up filling in a hole from his past and over the time that they were together, he grew to like who she was as well. For Malory, she first liked how Kael was like her, with a scar, and grew to like him more as how he treated her and how possessive he was. A plus to this relationship was that they were able to spend a lot of time together and got to know one another like brothers...Which obviously led to more then that later...(There's more but this is the basic principals of their relationship.)
For Anonym and Switch, Switch was plainly bored, not caring and with Anonym around, he was entertained and felt like he was living again. With Anonym, granted it took a while for her to completely fall for Switch, but she liked him around and loves what he does, so he also ends up doing the same thing with her as she does him. Similarites also helps this couple, as they are both reborn all the time and have big pasts, also very powerful. (There's more but this is the basic principals of their relationship.)
So my advice would be to have something that they can make the other feel special and happy. Add in a bit of easy fun and don't forget to add a connection and them wanting to be with that person more and more as the time goes by.
Other things to think about would be that when one is in love with another, the boundaries are losser. Forgiveness is a lot easier to obtain, happiness easier to precure, live feels shorter, no regrets and not wanting tomorrow to come (even if it's to say goodbye for one day). Dazing out, blushing and running when you don't normally run...Stuff like that...
...I think I've covered a lot...Did I miss something?