When Romeo fell in love with Juliet, he began changing. Before he met Juliet, the relationship he had with the Capulets was a troubled one. He was constantly in a brawl with a Capulet. From the moment he met Juliet, he is changed forever.
An example of this is found in Act III, Scene I. Romeo has just married Juliet. Only an hour has passed and he meets Tybalt. Tybalt is ready to fight Romeo. Romeo tries to convey to Tybalt that he does not want to fight. He tells Tybalt that he loves him.
Tybalt, the reason that I have to love theeTYBALT ROMEO
Doth much excuse the appertaining rage
To such a greeting: villain am I none;
Therefore farewell; I see thou know'st me not.
I do protest, I never injured thee,
But love thee better than thou canst devise,
Till thou shalt know the reason of my love:
And so, good Capulet,--which name I tender
As dearly as my own,--be satisfied.
Since Tybalt does not know that Romeo has just married Juliet, he does not feel the same way as Romeo. In fact he is ready to kill Romeo.
Tybalt kills Mercutio. Then Romeo has to honor his friend's death by killing Tybalt. After all is said and done, Romeo would not have killed Tybalt had not Tybalt killed Mercutio.