Whether or not I believe in the current characters HEA or HFN depends on a few things. It also depends on how well the author matched up the hero and heroine. An author can have all the ingredients in the novel that would make me believe these two belonged together. They have some shared interests. It could be as simple as the same t.v. show. Or that they both, deep down, want to save humanity or *insert a shared interest* There's chemistry between them. Maybe in some part of the process of falling in love they changed their status quo for the better.
I'm not one of those people who doesn't believe in love at first sight. I also don't get hung up on the timeline of when the characters first meet and when they fall in love. For instance, Agnes and the Hitman timeline was a week, MAYBE two, but I believed in their HFN.
So what do I believe this IT factor hinges on?
*WARNING THIS IS STRICTLY WHAT I BELIEVE IT MAY NOT HOLD UP IN COURT, UNDER A MICROSCOPE, OR ANY OTHER TESTS*
Do these characters not only compliment each other, but also challenge and/or encourages the other person want/need to be a better person. Could have someone else encouraged the change in the hero or heroine? I think this factor is why I'm drawn to write stories where the h/h are opposites, at least on the surface. There isn't a better way to change who or what you are than pairing up with someone who sees the world differently. You stop to smell the roses. The other person might say, "what roses I was drooling over the car that just passed."
Lastly, the theme of love makes you a better person is what resonates with me. Hence, my warning. A book where I don't believe in the HEA/HFN ending is dependent on the theme that resonates with me.
Why two people fall in love is as different as the MOMENT two people realized they were in love. It could be over a cup of coffee. The first date. A look across the room. Something about the other person moved their soul. Or, the other person cooked a fabulous meal just for that person in their life.
Love is one of those whimsical things that you can't describe, but you can show examples of it. Your examples may be totally different from mine.
Of course, I want to know what resonates with you? What characters are you drawn to read? What characters are you drawn to write?