By the end of Act I, Tybalt has confronted Romeo because...

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Romeo has angered Capulet, and Tybalt rises to defend his honor.
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Romeo, a Montague, has disguised himself to get into Capulet's feast.
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Romeo insulted Tybalt in front of all the Capulets.
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Romeo has been dancing with the woman Tybalt loves.

1 answer

By the end of Act I, Tybalt has confronted Romeo because Romeo, a Montague, has disguised himself to get into Capulet's feast. Tybalt is outraged that a Montague would dare to crash the Capulet party, leading to his confrontation with Romeo.