She passed the salley gardens with little snow-white feet. (4-5)
The minute we see little snow-white feet, we get the feeling that this story is going to end in the same way other stories about innocent little feet tend to end. Youth is usually depicted in this sort of innocent looking way, so the romantic clichés have a purpose here.
But I, being young and foolish, with her would not agree. (7-8)
When we're young and foolish, we tend to think we know it all. We don't like to agree with good advice because we think we know better. Of course, the speaker pays the price for his own arrogance and ends up full of tears.
But I was young and foolish, and now am full of tears. (15-16)
The only way for the speaker to really learn about life and love is to make the mistakes of a young person. No matter how foolish he was, the lesson he learns couldn't have come about any other way. Those tears are part of the whole process of growing up.