Explain what Romeo is telling Benvolio about what he is feeling in Act 1, Scene 1 of Romeo and Juliet.

After their greeting, Romeo asks Benvolio if that was his father he just saw leaving and Benvolio replies in the affirmative. He also tells Benvolio that his sad hours seem long and tells him that the reason for this is that he does not have that which would make the hours seem shorter. He is obviously referring to Rosaline who, he says, has rejected him. He says that he is out of favour with the one he loves.  

Romeo then goes into a passionate tirade about love being similar to hate. He juxtaposes the two based on the fact that Benvolio has just told him about the brawl that ended sometime before his arrival. Romeo says:

Alas, that love, whose view is muffled still,
Should, without eyes, see pathways to his will!
Where shall we dine? O me! What fray was here?
Yet tell me not, for I have heard it all.
Here's much to do with hate, but more with love.
Why, then, O brawling love! O loving hate!
O any thing, of nothing first create!
O heavy lightness! serious vanity!
Mis-shapen chaos of well-seeming forms!
Feather of lead, bright smoke, cold fire,
sick health!
Still-waking sleep, that is not what it is!
This love feel I, that feel no love in this.
Dost thou not laugh? 

Romeo is clearly lovesick and states that love, which is supposed to be blind, has seen a way into his will. It controls him. He mentions that the brawl has more to do with love than with hate, for he is fighting a battle with love. He uses a number of oxymoronic statements to illustrate his fight for Rosaline's love.

His paradoxical statements display the fact that he is utterly confused and distraught by the fact that he has to fight so hard to get Rosaline's attention and win her affection. His love is contrasted by her coldness. The love that he feels for her is not requited. He is miserable and asks Benvolio if he is laughing at him for feeling the way he does.

Benvolio says that he would rather weep at Romeo's unfortunate situation. Romeo then delivers a speech about this:

Why, such is love's transgression.
Griefs of mine own lie heavy in my breast,
Which thou wilt propagate, to have it prest
With more of thine: this love that thou hast shown
Doth add more grief to too much of mine own.
Love is a smoke raised with the fume of sighs;
Being purged, a fire sparkling in lovers' eyes;
Being vex'd a sea nourish'd with lovers' tears:
What is it else? a madness most discreet,
A choking gall and a preserving sweet.
Farewell, my coz.

He says that love does not follow the rules and that his pain places a heavy burden on his heart. He states that Benvolio will just worsen his condition if he tells him more about his own love for Romeo. Love to him is like smoke which is brought about by the vapour created from one's deep sighs. When love is cleansed, it brings a sparkle to lovers' eyes and if it is troubled, it brings about a sea of tears. He believes that it cannot be anything else. It is a brain sickness which hides itself, and is at once bitter and sweet.

Romeo further informs Benvolio that he has not been himself. When Benvolio asks him who it is that he loves, Romeo says that it is like asking a man who is sick with sadness to make his will. He declares that it is a woman that he loves. He then confesses that the one he loves is attractive and that she will not return his affections ('she'll not be hit with Cupid's arrow'). 

He furthermore states that the object of his affection is quite intelligent and extremely chaste. She refuses to respond to love's call and will not allow herself to be affected by sweet nothings or endure his affectionate looks or receive even his most valuable gifts. She has a wealth of beauty but Romeo feels that she is impoverished for she will die with all her beauty intact, instead of having profited from it.

In addition to all that, Romeo believes that in her refusal to show him any charity, she is wasting her beauty for it will not be treasured for all time. He says that she is too wise and beautiful to reward her own happiness by making him fall into despair. She has vowed to never indulge in love and that has made his life a living hell.

...She hath forsworn to love, and in that vow
Do I live dead that live to tell it now. 

Benvolio then advises Romeo to forget about her and look at other beautiful girls. It seems, however, as if Romeo is intent on being miserable, for he says that looking at others will only remind him more of her exquisite beauty. He uses an analogy in which he speaks about ladies wearing black masks so that others may guess at the beauty they hide beneath. It just makes others even more curious. He states that even one who is struck blind will forever remember the beauty he had seen. Furthermore, if he should perceive another beauty he will, once again, just be reminded of the fact that the one he loves is more beautiful. He then bids Benvolio farewell, stating that he cannot make him forget.

It is ironic that these are Romeo's closing words to Benvolio because it is not long after that that he does, indeed, forget. When he sees Juliet later at the Capulet ball, he is overwhelmed by her beauty and is completely lovestruck. Rosaline immediately becomes not even a distant memory. It is as if she never existed at all.