Dime por favor quién me mandó quererte.
Tell me, please, who told me to love you [why should I love you?].
Caption 1, Romeo y Julieta - Episodio 59Play Caption
Romeo wants to know who in the world asked him to love Julieta: Dime, por favor, quién me mandó quererte. "Tell me, please, who told me to love you." It's a rhetorical question. Nobody asked him to love her, so why should he?
Perhaps you are familiar with the verb mandar, meaning "to send." Many Spanish learners (and even many native speakers) are likely to be tempted to translate quién me mandó quererte as "who sent me to love you." But there is another meaning of mandar, which is "to order" or "to tell" (someone to do something), and this is the meaning that Spanish grammarians inform us comes into play when the construction is mandar + infinitive.
A Pedro lo mandé traer un litro de leche.
I told Pedro to bring a liter of milk.
If Romeo had wanted to say "Tell me who sent me to love you," he would have had to put an a before the infinitive, Dime quién me mandó a quererte. The construction mandar a + infinitive means "to send" (someone to do something).
A Pedro lo mandé a traer un litro de leche.
I sent Pedro to bring [back] a liter of milk.
Since the meanings are so close, it is only natural that in many parts of the Spanish-speaking world people use mandar and mandar + a indistinctly. In other words, they no longer differentiate between the two. Something similar is happening with deber and deber + de, remember? But it is a good idea to learn the rule while understanding that it doesn't always hold up. Like many other things having to do with rules and life!