Even more important than the two of them being best friends, they are teaching each other tolerance and acceptance. Sunshine definitely tests Monster's patience. She tests all of our patience. She teaches him self control. When he'd really like to haul off and slug her (and, honestly, most of the time, it would be justified), he models how to control his anger. "[Sunshine], you are making me so mad right now!" His self control is teaching her self control.
They also teach each other how you can be yourself and do your own thing while being part of a team. How the two of them combine imaginations is incredible. They let each other be who they are. So often kids want to only play something their own way and it has to be just so and if it's not, they don't want others playing. Monster and Sunshine are so different yet they can combine their interests to make each other happy. Sunshine will be dressed like a princess while having a sword fight. Monster will be pretending to be Merlin while taking care of Sunshine's baby dolls. Of course there are times they don't want to play with each other, and that's fine, too. They talk it out.
As Thanksgiving approaches, I think about how very thankful I am that my children love each other so much. I know their relationship will change as they get older, but they have a bond that can't be broken. They may have years that they don't want to spend time together, but I am confident that they will always be there for each other and lean on each other. I am so proud of the two of them and the relationship that they have built.