If love be rough with you, be rough with love...

Ay me! sad hours seem long.

~ Ever has it been that love knows not its own depth until the hour of separation. ~

Romeo: Is love a tender thing? It is too rough, too rude, too boisterous, and it pricks like thorn.
Mercutio: If love be rough with you, be rough with love. Prick love for pricking and you beat love down.

Romeo: I dreamt a dream tonight.
Mercutio: And so did I.
Romeo: And what was yours?
Mercutio: That dreamers often lie.

---

"Virtue itself turns vice, being misapplied,
And vice sometime's by action dignified."
Means:
The Friar is saying that all things have the potential for both help and for harm (virtue and vice). His soliloquy is while he is picking herbs for his potions, but the properties do not relate strictly to plants. No matter what good qualities something (or someone) possesses, if they are misused for any reason, the result may be more harmful than helpful. You've heard of "too much of a good thing...", right? This idea will fit both Romeo and Juliet as the play progesses (foreshadowing) because, for them, too much love, no matter how wonderful it may be, once it is "misapplied" (becomes obsessive) will turn out to be disastrous. For them, "virtue itself turns vice" and they both commit suicide for love.

"What must be shall be."

"He that is strucken blind cannot forget
The precious treasure of his eyesight lost."
Romeo and Juliet, 1. 1

I have more care to stay than will to go.
Romeo and Juliet, 3. 5
Thank me no thankings, nor proud me no prouds.
Romeo and Juliet, 3. 5