How is Romeo's attitude toward love presented in Act 1, Scene 1 and how does he use oxymoron to express his mixed emotions?

Romeo describes the fickle nature of the feud and the complex nature of love.

When Romeo sees the aftermath of the marketplace brawl, he understands what happened immediately. He is a Montague and knows about such brawls. Romeo is a sensitive young man, though, and knowing what happened upsets him. He expresses the depth of his emotions through oxymoron, a figure of speech involving two contradictory or opposing elements such as "loving hate," as can be seen throughout his following speech:

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 . . . (Act 1, Scene 1)

Romeo has a point. The brawls, while serious, come from a place of ridiculousness and vanity. There is no substance behind them. People attack each other for no reason, because they like to fight or because they are blinded by the fact that the other family is the enemy.

Romeo does not have personal enemies. He is a lover, not a fighter. Poor Romeo is suffering from his unrequited love of Rosaline. He is in a low place. When he sees the violence, he ponders the connection between love and hate. He speaks to Benvolio, his cousin and friend, about the meaning of and pain associated with love.

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 . . . (Act 1, Scene 1)

Love can be fickle. Someone can be in love with you one day and no longer in love with you the next. That is just part of being young, or part of being human. Romeo is saying that love can be temporary and frustrating, even though it can also be powerful.