I have been reflecting for a long time on what it means to really follow Christ in our changing world. What does discipleship look like when it really takes hold of a life? I believe that discipleship is the priority in the Christian life, so why is it so important? (Yup, I said, "the priority.")
I'll begin here with a few thoughts about what is probably the bottom line, the ultimate reason. It is true.
The whole story is true. Every bit of it really is the way God said it is. He really did create the universe and place humans created in his image in it to tend to it and be in relationship with him. He really did reveal himself. He really did come in person of Jesus Christ, born of a virgin, lived the sinless life, willingly died on the cross, conquered death and walked out of the grave, ascended into heaven, and sent the Holy Spirit.
Every promise Christ spoke about the reality and power of the Kingdom of God is real. None of it is bubbles and smoke. None of it is intended for only the super-spiritual. All of it is for whosoever will.
The uncomfortable things spoken about the brokenness of the human condition by God are all unavoidably true. It has been said that the doctrine of original sin is the most unlikeable of Christian doctrines, but also the most obvious. Humanity, without Christ, is hopeless - literally.
Now the reason this is an answer to the question of why we ought to be disciples, is that humans are conditioned to believe what is true. It is in our very nature to seek out what is true about ourselves and the world around us. Reality is unwavering and we must, in the end, live according to its terms. This is so ingrained within the human nature that psychology has at least a handful of diagnoses for people who refuse to deal with what is real and true.
We actually live as if 2+2 really does equal 4. Whether we acknowledge it or not we live according to the laws of reason. Human communication would be nearly impossible otherwise. We are ultimately uninterested in false theories of gravity and chemistry because we know they are only useful as curiosities or as bad ideas that help us understand the goodness of the true theories.
The same is true between people. Unless we have reason to believe otherwise, we assume, within reason, that people tell us the truth. If you don't believe me, try going through a day believing everything everyone says to you is a deliberate lie. You will not get far. Even when we interact with strangers, we are built to tell and believe the truth.
There is no good reason not to apply this same standard to ultimate things - to the things of God, life, and salvation. We live best in reality when we are in accord with it, and if God created all things then, we live best when we live in accord with that God. And discipleship is learning to live in accord with God and his creation. When we learn to follow Jesus Christ in his way of life, we are learning to think and act the way God did when he was here on earth in human flesh. Truth himself came and walked among us (John 1:14, 14:6), now we follow.