depends on the potions... but what your probably talking about is the one that he gave to Juliet, and that's because he is a friar, lives a sedentary lifestyle in which he would understand all the plants and such, because at the time friars were also herbalists, and you use herbs to make potions...
He's good at Potions.
Friar Laurence married off Romeo and Juliet.
Friar Laurence is the friar who marries Romeo and Juliet.
Friar Lawrence married Romeo and Juliet in his cell.
Juliet, Romeo, Tybalt, Nurse, Benvolio Or . . . Juliet, Romeo, Friar Lawrence, Nurse, Capulet Or . . . Juliet, Romeo, Friar Lawrence, Capulet, Tybalt Or . . . Juliet, Romeo, Capulet, Mercutio, Tybalt Or basically Romeo and Juliet and any three of Nurse, Friar, Capulet, Mercutio, or Tybalt.
Romeo, Juliet, and Friar Lawrence are in Friar Lawrence's cell.
Friar Lawrence wants Romeo to do different things depending on what part of the play you are talking about. When they first meet, the friar tells Romeo to go slowly in his relationship with Juliet (as if!).
Friar Lawrence was the priest in Romeo and Juliet
The friar.
Friar Lawence
It was both Romeo's and Juliet's idea to marry eachother.
Romeo asks friar Laurence to marry him and Juliet in secret