Juliet is nervous about talking the potion. She worries that it might be poison, or that she might be lying with ghosts in the tomb, wake too early and be scared. She finally seems to decide that Romeo is relying on her, and she has to be strong.
When Juliet is about to take the potion, she is very nervous. First, she is not completely confident it’s not poison. She is afraid Friar is trying to poison her because he does not approve of their marriage. Yet she concludes that he wouldn’t poison her:
I fear it is; and yet methinks it should not,
For he hath still been tried a holy man.(30)
I will not entertain so bad a thought. (enotes etext p. 94)
She decides that he is a holy man, and so he wouldn’t do it.
She then worries that she’ll wake before Romeo comes, and might suffocate or be attacked by her family’s ghosts, especially Tybalt. It is actually the idea of Tybalt’s ghost coming for Romeo that gives her the courage.
O, look! methinks I see my cousin's ghost
Seeking out Romeo, that did spit his body
Upon a rapier's point. Stay, Tybalt, stay!
Romeo, I come! this do I drink to thee.(60) (p. 94)
Juliet alleviates her concens by thinking of Romeo. So in the end it is Juliet’s love for Romeo that convinces her to take the potion and follow through with the plan. She is able to gain the courage, and it is almost like she is trying to protect Romeo by taking the potion then, just as she is taking the potion in order to marry Romeo and be with him, protecting him from his banishment.