I'm busy re-writing this love scene, so declarations of love are much on my mind. Recently I've read some love scenes that didn't work for me. My main problem was not that characters declare love for each other, it was the circumstances of how love was declared.
1. The announcement out of the blue.
The reader doesn't see the announcement coming. Now, this could work if the scene is in character A's viewpoint, and Character B makes the declaration. But then you'd expect A to be surprised, even if they were delighted. If they just go, 'I love you too,' without having any other expression (internal thoughts or external dialogue) it makes them come across as a) emotionally flat lining and/or b) extremely conceited as it comes across as if they expect people to declare love.
2. The characters who hate each other
I know, I know, it's a favourite from Pride and Prejudice onwards - they meet, they don't like each other, they fall in love. But in P&P we see both Lizzie and Darcy's development, from his early crass declaration and her refusal, to his second proposal and her acceptance. We know she has changed her opinion of him almost before she does. This takes time...it simply doesn't work if that morning the characters were at each other's throats but by elevenses they are cuddling up to each other and looking gooey eyed.
3. The oblique declaration
When I was about 18, an old school friend I was visiting at university told me that we were destined to be together, and that later on we'd get married. Perhaps I was suppose to blush, and agree thereby showing my own feelings. Given there was no romance at all between us up to that point, and from my point of view felt there never would be, I smiled politely and didn't say anything. Not seen him since. But he was young. Do adults go round making these sort of declarations without even having had some indications that their affections are seriously reciprocated? I don't think so. In other words, the reader should be also aware of it.
4. The immediate declaration
A close friend of mine walked into a university lecture theatre and was spotted by another student who turned to his friend and declared, 'that's the girl I"m going to marry.' He managed to meet her, chatted her up, they went out with each other, and did indeed marry - and have been married for more than 30 years. The point is, he didn't tell her about his feelings until they'd been together for some time. Again, do sensible adults meet someone they like, and within a few minutes declare their feelings? And if they do, do we as readers believe that those feelings are genuine and well founded?
So, what makes a believable declaration of love? Well, the opposite of the above points, in my opinion. Characters don't have to like each other from the start but the writer must give them time to change. They ought to be circumspect in the timing of their declaration, and if they do behave impulsively they out to be aware of that (unless they are either 15 or it isn't going to be a realistic love at this time, though again it could develop in time).
And above all, the reader must be along for the ride. What's the point of having a romantic story if the reader doesn't live it vicariously?