This is a very good question and one, I think, that you will be able to solve with a bit more scrutiny of the quote, its dramatic irony, and the relation of both to one of the major themes of the play: time.
Both characters, Romeo and Juliet, act with great haste in this play. In fact, Shakespeare emphasizes the haste of the young lovers by compacting the action into a few short days. The swift passage of time and the hasty actions of especially the young lovers leads to the tragic events of this play.
So, in considering a thesis statement for an essay based upon Romeo's line to the drugged (but not dead) Juliet, consider how all of their actions have lead to this moment. If Romeo would only pause for a moment or two, he would be greeted with the waking of his love, rather than lying in a pool of his own blood at her heartbroken feet. If he would only pause long enough to consider his own words and how they are cluing him in to the truth of Juliet's physical state, then the events of the play could still avoid their tragic conclusion. This line signifies, if you will, the last chance Romeo has to "get it right."
Here is an example of a thesis statement (NOTE: PLEASE CONSTRUCT YOUR OWN WORKING THESIS STATEMENT. THIS IS AN EXAMPLE ONLY.): Romeo's line, "Why art thou yet so fair?" of Act V, scene iii, indicates the culmination of a series of actions that have, by their haste, constructed the tragic events that have led to the lovers' own downfalls.
For your thesis, it is the connection between time/haste as an important theme in the play, and the headstrong and hasty actions of Romeo and Juliet as they try to manipulate/control the events of the play, that you will want to emphasize, in order to create a "provable" thesis.
For more on the thematic issue of time and Romeo and Juliet, please follow the links below.