Thou blind fool, Love, what dost thou to mine eyes, (1)
The poem starts by addressing Love as a god. This tells us a few things right off the bat. First, we know that the speaker thinks of Love as something outside of himself, some force that swoops in and messes up his life. Also, because of what we know about gods, this reference shows us that Love is very powerful. From the first line, can we tell whether the god of Love is using his power for good or for evil? Not exactly—the speaker has just begun to ask the question, which won’t even be completed until the next line. But the abrupt, angry tone of the opening, "Thou blind fool, Love," suggests that it’s not going to be pretty.
That they behold and see not what they see? (2)
Now, the speaker finishes asking his question. The problem seems to be that Love has taken away the speaker’s ability to understand reality. What the speaker is saying is that his eyes look at something, and they don’t understand what they’re looking at. In a way, you could say that the somewhat confusing way the speaker has phrased this is a sign of the confusion that Love has brought to his life.
They know what beauty is, see where it lies,Yet what the best is take the worst to be. (3-4)
Did you notice that the previous line of the poem still didn’t make it very clear what the speaker was talking about? Sure, he made it clear that he thinks Love has taken away his ability to understand reality, but did he bring in any evidence to back this up? Not so much. In these two lines, he makes things a little bit clearer: his eyes are experienced, they know what beauty is, but they can’t stop looking at the "worst" person in the universe and thinking that she (we're assuming it's a she) is the "best."