When John hesitates, Jesus says the baptism should happen "to fulfill all righteousness" Matthew 3:15. John then consents and baptizes him Matthew 3:15–16.
In Matthew 3:13–15, Jesus travels from Galilee to the Jordan River and comes to John specifically in order to be baptized. John resists, objecting that the roles seem reversed because John feels he needs baptism from Jesus. Jesus answers by asking John to allow it “now,” explaining that this is the fitting way for “us” to carry out what God calls right. This scene follows John’s public ministry of calling people to repentance and baptizing at the Jordan, where he also speaks about the stronger one who is coming after him Matthew 3:1–12.
Matthew presents Jesus as acting deliberately: he travels from Galilee to the Jordan with the stated purpose of being baptized by John (explicit). The scene also shows John’s reluctance. John thinks the request reverses the proper order, because he says he needs baptism from Jesus (explicit).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Matthew 3:15, then read the surrounding section so the answer stays connected to the passage's flow.
15But Jesus, answering, said to him, "Allow it now, for this is the fitting way for us to fulfill all righteousness." Then he allowed him.