This was a project me and a bunch of other people were doing on devaintarts, it's still on going and we keep adding more reasons. But I'll post the 50 reasons we got so far. :)
1. They grew up together
2. England grew his hair out like France
3. He got in trouble doing so
4. France got worried thinking England got eaten by a caterpiler
5. France helped England with his hair
6. He fixed his hair back to the way it was, saying he looked better that way
7. They raised Canada together
8. England put his arm around France
9. They are always together
10. They work together a lot
11. They comfort each other
12. France asked England to marry him
13. They tease each other
14. the translator in Paint it White said "They loved eachother sexually"
15. The Manga's character role call (second book) both France and England's descriptions had something to say about eachother.
16. France cared and tried to look after England when he got really sick
17. He yelled at America when he called England a "jerky limey"
18. England didn't want France to catch his cold.
19. France thinks England is cute
20. France brought England a dress
21. England agreed to wear it
22. In Paint it White they walked out of the jungle togather
23. They are seen together a lot threw Paint it White
24. They get paired up together in Paint it White
25. England let France conquer him in 'Crossing Through the Year 1000'
26. J.Micheal Tatum (France) and Scott Freeman (England) thinks its canon and totally accurate
27. France asked England if he wanted to join him in a "Masturbation Marathon"
28. France is the only one that knows England can't swim
29. The Entente Cordiale, the treaty between France and England, a treaty of friendship and peace that still lasts today
30. They fight like an old married couple.
31. France tried to get England to strip with him in the Olympic episode
32. They always have each others back.
33. They fell out of the ship into the ocean; England can't swim; only France knows this; ergo, France carried England to shore.
34. They keep each other company on Christmas
35. They spread rumors together
36. France is the only other nation so far who can see England's fairies and mystical creatures
37. UK's motto is written in French
38. The legends of King Arthur are very popular in France
39. "Arthur" is one of the top French baby names despite not actually being a French name
40. They say they hate each other, but they still stay together
41. You can pair them in any point in history
42. They spent Halloween together
43. They call each other friends
44. They go out for drinks together
45. They sat close together in Catalia
46. They stay at each others houses
47. They go out to eat together
48. According to Himaruya's Comic Birz,they have a very spicy relationship
49. England said France could accomplish anything if he put his mind to it
50. England uses France's chef to cook for him