"Loveliest of Trees" first appeared in a book called A Shropshire Lad (1896). This is because it was full of poems about a whole bunch of Shropshire lads, including the speaker of "Loveliest of Trees." So he's a Shropshire lad—bully for him. What does that mean? It means he's a fellow from Shropshire, a county in western England that is right along England's border with Wales. We tell you a little more about Shropshire over in our "Setting" section, but for now we're gonna cover the lad part.
Now, a "lad" is just a young guy. We usually tend to think of lads as a bit younger than our 20-year old speaker. However, when compared to, say, a 70-year-old, the speaker is definitely still a lad. Okay, so he's a young guy, he's got a horse, and he has the luxury of taking nice rides out in the open where there are lots of cherry trees. This may mean that, financially, he's doing okay (no daily grind in some factory for him).
And not only is the speaker a young guy, he's a young guy on the cusp of adulthood. All the spring imagery in this poem (Eastertide, blooming trees, etc.) suggests that the speaker is in the spring of his life—about to step out into the world as an adult. He knows this, but he also realizes something that most lads his age don't: that he won't be young forever. You could almost say that he's a young guy who has just had an eye-opening experience, a wake-up-call telling him that life is short and he better get busy living. That realization gives him a pretty rare and mature perspective, one that most folks don't come to until their much closer to 70 than 20.