What threats already exist to Romeo and Juliet's love in Act 1?

In Act I, Romeo himself is very much a threat to his love for Juliet. He doesn't intend to be, of course, but his immaturity and youthful impetuosity mean that he doesn't yet fully understand what true love is all about. His infatuation with Rosaline, for example, is largely an expression of puerile lust. It's a measure of Romeo's immaturity that he's prepared to pay Rosaline for the privilege of sleeping with her. If that's how he feels about the opposite sex at this stage, then it doesn't augur too well for any future relationships he may have.
But of course the biggest threat in Act I to the star-crossed lovers is the biggest threat in the play as a whole. According to the standards of the time, neither Romeo nor Juliet are to be regarded mainly as individuals; they are members of ancient aristocratic families, duty-bound to protect the honor and dignity of the clan. Love is seen as a gross self-indulgence in a social environment where marriages are conceived of primarily as strategic political alliances.
The prevailing standards dictate that such marriages are to be arranged and that prospective husbands and wives must yield to the wishes of their elders and betters. At the best of times, this would make things hard enough for Romeo and Juliet. But with their respective families at each other's throats, their incipient love faces a seemingly insurmountable hurdle.

There are a number of threats to Romeo and Juliet's love in Act I. First of all, there is the feud between the Montagues and Capulets. This bitter divide between the two families, which frequently erupts into violence, threatens their love because it casts Romeo and Juliet as sworn enemies.
Secondly, Rosaline also threatens Romeo and Juliet's love. In Act I, Scene I, for example, Romeo is heartbroken because Rosaline does not reciprocate his feelings. However, by swearing that she will never love Romeo, Rosaline also paves the way for Romeo and Juliet's union, thereby switching this threat into a favorable condition.
Finally, Paris also provides a threat to Romeo and Juliet's love. In Act I, Scene II, for instance, the reader learns that Paris has already asked for Juliet's hand in marriage. If she were to go ahead with a marriage to Paris, she would never have the opportunity to be with Romeo and fall in love with him.