@Shiroumiso A bit of a late response but here goes anyway:
When your (childhood/best) friend becomes your lover you essentially treat one person as someone with two roles (for a lack of a better term). The role of a lover and a friend may have huge overlaps, but there will always be things that are/must be appropriate as friends or as lovers only. There are things you can agree on as friends, but not necessarily as lovers, and vice versa. This further gets complicated since the two of you are both friends and lovers.
A naive solution would be to try to separate these roles, wherein you try to act as friends in certain occasions and lovers in certain occasions. But this won't always work. The other role will always get involved and affected, one way or another.
So to deal with that you have to consider the person as a whole: as both friends
and lovers. An intersection. You risk messing things up if you only treat them as a friend ("Don't you love me?") or only as a lover ("You're not the same person I fell in love with anymore..."). What's even harder is the messing things up part. You must be considerate for either the friend or lover role. A union. If you mess up one part, you mess up all. Complicated arguments/fall outs happen and what not.
Of course, all of this can be avoided if you managed to successfully communicate things with your partner and set clear boundaries/expectations and what not. If you end up on a clear, mature romance, then congratulations. But since we're talking about childhood/best friends -> lovers, chances are it would be your
first romance. Not a whole lot of experience there.
I know I'm ruining it all for bringing the "real life" into this fantasy romance fluff manga, but I just really want to point it out.