Romeo is depressed at the beginning of the play because his love for Rosaline is not returned. Rosaline has sworn off all men. Of course, as you read, you realize that this is not real love because the moment he sees Juliet he forgets all about Rosaline. Romeo's fatal flaw of impulsiveness dictates his heart.
Benvolio's attitude towards Romeo is that of concern. Benvolio wants to help Romeo get over Rosaline and explains to him that when he saw Rosaline she was alone, so there was no one to compare her beauty with. He encourages Romeo to go to the Capulet ball where he will see Rosaline compared to other girls and will then realize she really isn't that great! Benvolio wants to help Romeo get back to normal. He doesn't like seeing him depressed.