Note: contains relationship spoilers from The Hunger Games
What is romance?
a feeling of excitement and mystery associated with love.
Most books include romance. At least a little. Just think back to your childhood, the picture books where the princess is locked up and a prince rescues her. Nowadays this is usually called stereotypical romance and featured in many books.
A lot of Young Adult books contain love triangles, a concept which is both loved and hated by readers. Most love triangles (eventually) resolve, but the result is often nowhere near as good as it could've been.
For instance, The Hunger Games. Katniss, Peeta and Gale. We end with Katniss and Peeta married and with children. Where is Gale? Relocated to District Two, far away from Katniss and Peeta. Personally, I'm an Everthorne shipper so the ending really didn't appeal to me. The final chapter didn't mention Gale at all, which was a huge let down.
Also: love triangles generate a lot of conflict within readers as they decide who to back. The "losing" team will generally be very disappointed and unhappy with the ending. I like the drama love triangles create, but I think the resolution is a huge part. If it's not done well, the book/series can be ruined.