Why are romeo's parents worried about him

Many a morning hath he there been seen, 
With TEARS augmenting the fresh morning dew. 
Adding to clouds more clouds with his deep sighs; 
But all so soon as the all-cheering sun 
Should in the furthest east begin to draw 
The shady curtains from Aurora's bed, 
Away from the light steals home my heavy son, 
And private in his chamber pens himself, 
Shuts up his windows, locks far daylight out 
And makes himself an artificial night: 
Black and portentous must this humour prove, 
Unless good counsel may the cause remove. 
BENVOLIO 
My noble uncle, do you know the cause? 
MONTAGUE 
I neither know it nor can learn of him. 
BENVOLIO 
Have you importuned him by any means? 
MONTAGUE 
Both by myself and many other friends: 
But he, his own affections' counsellor, 
Is to himself--I will not say how true-- 
But to himself so secret and so close, 
So far from sounding and discovery, 
As is the bud bit with an envious worm, 
Ere he can spread his sweet leaves to the air, 
Or dedicate his beauty to the sun. 
Could we but learn from whence his sorrows grow. 
We would as willingly give cure as know. 
Enter ROMEO 

Both are worried about him because he is depressed, sleeps all day and is out all night, dislikes the company of others and at the end of all is won't tell them or anyone what is wrong.  i hope this helps :)

The authors choice of words and language