“…they stumble that run fast.”
Friar Lawrence is trying to express the fact that Romeo and Juliet are rushing into things, and when you rush into things you should be prepared to fall or stumble. Also their feelings are coming like a tidal wave and overwhelming them, which can result in your judgement being clouded.To get married is committing to someone; before you do that it is best to get to the person you are planning to spend the rest of your life with. Romeo and Juliet are jumping right into after knowing each other for a day. Romeo and Juliet aren’t taking a step back to see the events that might unfold. Also this line is foreshadowing the Romeo and Juliet’s fate. Friar Lawrence is concerned about the consequences with the expeditious decisions that are being made due to their affection.