Jesus and the disciples continued in their ministry when they encountered a blind man from birth. The disciples asked Jesus if the blindness was from the man's sin or parents. But Jesus proclaimed that neither the blind man nor the parents sinned (9:3)! Thus, contrary to some people's claims, not all bad things that happen to us are because of our sin or even our parents' sin.
In fact, John 9:3 can't necessarily be used to determine who caused the blindness! At first glance, the Greek word for "that" seems to convey cause-effect of blindness/healing, but that does not fit with the context or even God's character. In fact, there are viable alternate interpretations that are better because they fit the context better and provide better consistency to God's character. According to the 4b definition for Strong 2443 of Thayer's Lexicon, the Greek word for "that" paired with a subjunctive can be interpreted as a command (Bible Hub). With that meaning, Jesus is calls the disciples to let God work in him (rather than bicker around on who caused what). Additionally, considering that the punctuation in the original Greek text is technically non-existent, David Guzik and Tom Constable talked about how a possible way of punctuation is "But that the works of God should be revealed in him, I must work ..." (Blue Letter Bible, Study Light). Guzik's and Constable's phrasing maintains the meaning of Strong 2443 as "so that".
Both of the meanings are a better match of context because Jesus talked about the importance of him working in 9:4-5 (rather than discussing who else could cause the blindness). Jesus says it's importance for him to work the works of God because he has the opportunity to work and that the time for people to do works of God is limited (9:4). It's likely that Jesus is referring to death as night; the time of death, while different for different people, is a time where people can not work. Additionally, Jesus proclaimed that he is the light of the world while he is in the world (9:5). The point of a light is to be shined, so Jesus wants to make best use of his purpose while he is still on Earth because eventually, Jesus would go on the cross and ascend to heaven.
Some of us have questions similar to the disciples' questions, and they can be time-consuming and even stressful. But in the midst of uncertainties, Jesus wants to put more priority on what can be done for God's work to be seen. For us, there are plenty of ways to get involved in service (not just in church programs). But for Jesus, this included dying on the cross and raising himself from the dead for our behalf as well as showing himself and his character so we (and our friends) can receive everlasting life.