In the play "Romeo and Juliet" by William Shakespeare, the author describes a son who is perplexing to his father as he is, as youngsters today might say, acting like an emo! So instead of enjoying the gorgeous Italian sunny weather and being full of the joys of Spring, Romeo's love-sickness is causing him to reject the light in favor of the dark. This theme of Light and dark is important in the play. The description of how Romeo makes his chamber dark and cold and forbidding on a bright day foreshadows the way he is to die early and be entombed in a mausoleum. It also shows a serious dark weakness of his character - negativism and despair.Even a grove of trees would be shady.
Montague is saying that Romeo is being totally emotional.
Montague reports that Romeo has been seen pouting in the mornings and then he goes into the house as soon as the sun is shining and makes his room like night... dark. He closes the windows, door and drapes and won't let anyone in. He wonders what the deal is with his son.
Benvolio too wonders and asks if they've asked Romeo about it.
Motague reports that they have, but it does nothing. Many folks have, they also find that Romeo is acting like he's almost dead, he's so depressed.
Benvolio commits to find out what's up.
Basically, Montague is saying that his son Romeo has been acting all pathetic and love sick.
He says that Romeo has been seen in the woods quite often. Romeo has been crying (adding his tears to the dew) and sighing (making more clouds with his breath).
Montague says when the sun comes out and Romeo should be happy (because the sun is "all cheering), he instead runs home to bed and draws the shades. Please note that "heavy" here means depressed, not fat.
Montague says that unless someone snaps Romeo out of it, there will be trouble.