Reasons:

1. Because they have the most chemistry on the show.
2. Because both of them sing!
3. Because we'd kill to hear a duet.
4. Because Leighton loves Ed's scarves.
5. Because brunettes have more fun.
6. Because we look forward to their scenes the most.
7. Because they ARE Blair/Chuck.
8. Because they make us anxious for a new episode.
9. Because they play our favorite characters on the show.
10. Because we just can't get enough of them.
11. Because didn't you know? They're hot!
12. Because Leighton tugging on Ed's scarf is so cute.
13. Because we bet they loved filming the limo scene.
14. Because just looking at them makes our day better.
15. Because their awesomeness was unexpected.