Similarly to a priest, a friar ought to feel like a spiritual guide towards any person. This is especially true of friars who are also ordained priests, as Friar Laurence is, which we know from his ability to perform sacraments, such as shrift and wedding ceremonies. Hence, Friar Laurence feels fatherly towards Romeo. Likewise, Romeo sees Friar Laurence as a father figure. In addition, Friar Laurence views Romeo as very young and impetuous, while Romeo sees the friar as someone he can go to for help and advice. Shakespeare especially portrays their relationship through his use of diction and figurative language.
We can especially see that Friar Laurence treats Romeo as a son through the word choices in the ways he addresses Romeo. For example, he calls Romeo "[y]oung son" and "good son," which are both fatherly terms of endearment (II.iii.34, 48). In addition, Romeo reciprocates the sentiment by referring to the friar as "my ghostly father," meaning my "holy" father (46). Since holy fathers are seen as father figures, just like God the Father, in that they lead the spiritually young, this term of address is evidence that Romeo sees the friar as a spiritual father, or leader, and thereby as also a father figure.
Furthermore, Shakespeare even makes use of figurative language to portray Friar Laurence's feelings or opinion that Romeo is still very young in mind and impetuous. When Romeo protests that dropping loving Rosaline and loving Juliet instead is the right thing to do because Rosaline did not return his love while Juliet does, Friar Laurence counters with a metaphor to say that Rosaline was wise not to return his affections, as we see in his lines, "O, she knew well / Thy love did read by rote, and could not spell" (90-91). To say that Romeo's "love read by rote" is to say that his love read by memory. Friar Laurence is making an analogy between Romeo's love and a child who still cannot read but is pretending to, thereby showing us that the friar believes Romeo is far too young and impetuous to know what love really is. However, despite the fact that the friar sees Romeo as young and impetuous, it is also clear that Romeo feels the friar is someone he can trust and confide in. If Romeo did not feel he could trust Friar Laurence, he would not have told the friar of his love for Rosaline.