Hello, I'm Kate. I really enjoyed reading this. You did a great job. I love reading stories that are well done grammatically! If you continue this story, please PM me. I want to know what she says!
So, as far as I could tell, you didn't have any grammar mistakes; if you did, they were small typos that I am sure that you could fix. Nevertheless, I have a few suggestions that you may want to consider.
I didn't like how I didn't get to know the girls name until the very end, and I wouldn't have known Triston's name without the title! I know this is the beginning, but characters names are essential.
Also, I was confused at exactly their positioning outside. Were they standing up or sitting on a tree? Just a thought that you should try and make sure is clear when you are writing.
Overall, you did a great job. I can't wait to read more.
Keep writing,
Kate
"Sometimes it is said that man cannot be trusted with the government of himself. Can he, then, be trusted with the government of others?"
Triston stood back on the generous arms of the tree that, in that moment, seemed so much wiser than we ever would be.
I think you need to explain the tree more. In what way does is it wiser than your two characters? What does it do that makes your character point out the tree while a wonderful proposal is being made?
What is the tree?
Wiki answers: A tree often represents life and small, steady growth.
I think by this definition, I know what you are saying about the tree. The tree is wiser because it knows a slow and steady growth pays off, something your two characters do so obviously not know because they are getting married at a young age.
I don't know if this is what you meant, but I had to go and look that up. I suggest that you act like you did this on purpose if you didn't, and hint to readers what the tree symbolizes. Because not many readers would go and stop to look up a random symbol, because not many readers will be trying to pick out tiny details to help you with your story.
"You, who have all the passion for life that I have not? You, who can love and hate with a violence impossible to me? Why you are as elemental as fire and wind and wild things..." — Gone With the Wind