Because those two need to be together. My sister Riah and I experience this feeling very often. We're the queens of Unpopular Ships that Frequently Make No Sense (Finnick/Katniss, Thranduil/Tauriel, Eric/Tris, Daryl/Beth, you name it we ship it first class) and therefore our ships usually never get out of the harbor. This is probably why we love The 100 so much - because Bellarke is actually a thing. It's borderline canon. It's a Thing. Capital 'T.'
The best definition for shipping a couple is, "It's like being in a relationship with a relationship." And, like any relationship, it has stages.
STAGE ONE: INTEREST
You see Person 1 and Person 2 and you think, "Hmm. There's potential there."
STAGE TWO: IT HITS YOU
They look at each other. Or better yet, they speak to each other - and you know it. They're destined. Or at least they'd better be.
STAGE THREE: YOU SHIP IT
STAGE FOUR: SO, UM, HAVE YOU EVER WATCHED/READ THIS?
You should, because Person 1 and Person 2 need to happen and I need you to feel this with me, okay? OKAY???
STAGE FIVE: RE-WATCH AND ANALYZE
"Did you see how he looked at her right there? Did you SEE IT?" "She totally went back for him." "He didn't have to be nice to her right there." "Oh my word, she asked for his help!!!"
STAGE SIX: REALLY HATE ON THAT ONE CHARACTER WHO MIGHT RUIN YOUR SHIP.
Because they serve no purpose other than to get between Person 1 and Person 2.
STAGE SEVEN: WATCH ALL THE INTERVIEWS.
There has to be some kind of sign, right? They'll talk about this ship? Or it's been written about somewhere? Or somebody's made graphics? WAIT! FANVIDEOS!
STAGE EIGHT: YOU DRAG SOMEONE ON BOARD WITH YOU.
And it's glorious.
STAGE EIGHT (ALTERNATIVE): NOBODY WILL GET ON BOARD WITH YOU.
And it's awful. What are you supposed to do with yourself!??
STAGE NINE: SOMEONE SUGGESTS YOU GO OUTSIDE OR DO SOMETHING.
STAGE TEN: YOU GO OUT AND DO SOMETHING.
Everything reminds you of your ship. You realize you're officially past help.
STAGE ELEVEN: SOMEONE SAYS THEY HEARD YOUR SHIP MIGHT NOT BE CANON.
STAGE TWELVE: The series ends. Or you need to wait for the next book. Or your show goes on hiatus.
STAGE THIRTEEN: PEOPLE TALK ABOUT OTHER SHIPS.
...Why? We don't know.
STAGE FOURTEEN: YOU FIND ANOTHER OTP.
Lather. Rinse. Repeat.
BONUS STAGE: SOMEONE ASKS YOU WHAT SHIPPING MEANS.
And you just...