Joe summed it up very well! The spring is seen as the season of renewal when the trees and animals will look to... multiply haha. So it is linked to love in human beings. In addition, it is a common trope in poetry to use the seasons as stages in a person's life, so spring is associated with youth. A crude interpretation would be: "In the spring, a young man will be thinking about love"

Lord Tennyson was saying that after the cold, lonely winter, when it warms up and people venture out into the inviting spring, young men spend a lot of time thinking about girls and getting a girlfriend. I'm sure someone could offer a more detailed (or more elegant) explanation, but that is the gist of it.