First Impression of A Dream

I had to reread the poem a few times in order for me to grasp some what of an idea of what the poem is about. From what I could comprehend it was talking about how everything that we see is a dream, the second part of the poem made me feel a little sad because the author is talking about how endless the sand is but all he wants to do is try and save it from drowning and being taken away by the sea. The first half of the poem talks about someone departing from the author, I presume, and it sounds sad when I read it out loud because the person is saying that them meeting each other and their days together were just a dream, but them departing is so real and vivid. It made me a little confused because I can't really understand why the author says that everything is just a dream, when we experience everything in real life. Does all of this go back to the question of whether or not everything around us is just a figment of our imaginations? Are we even real? In order to distinguish my dreams from reality, I try and figure out if the situation, whatever it was, would actually be able to occur in real life- and if it would be too far fetched to be "real" then I deem it as something that my mind made up or one of my dreams.