In Act 4 scene 1, why does Juliet let Paris kiss her?Please help :)
She only lets him kiss her on the cheek.
This occured right after the Nurse turned on her and her parents are forcing her to marry him. She is at the Friar's cell and realizes that at least for now, she has to keep up the appearance of following through so she doesn't get in any more trouble with her parents.
As you watch the friar's body language in this scene, you can tell he knows that something strange is going on. So your basic answer is to keep up the appearance of everything going according to the Capulet's plan.
When Paris leaves, she explodes!
In short, to make Paris hurry up and go away. The last thing Juliet wants is to prolong this conversation with Paris which will only keep her from discussing Romeo's banishment privately with Friar Lawrence.
In this scene Juliet is an emotional wreck; she goes so far as to suggest that she will kill herself is Friar Lawrence cannot mediate the situation. Dealing with Paris, even listening to Paris's attempts to woo her, are too much. Her mind and her being at this point are focused on only one thing: figuring out a way for her and Romeo to be together.