I read once that the reasons given were legally allowed reasons to put off military service. In other words, in most circumstances, these are valid and understandable. Jesus denies them, not to be mean, for we've seen that Jesus is anything but mean, but Jesus is trying to help people see what the totality of commitment means. We're not accustomed to this kind of commitment -- we think in terms of what's convenient, and we do what we can, when we can. Most people understand when things get put off.
But Jesus is showing us how consuming the Kingdom of God is. Scripture also tries to show us that when we truly grasp the Kingdom in the depths of our hearts, we won't want anything to stand in the way of our commitment. This is what allows Paul to speak as eloquently as he does from prison. He's overwhelmed with the Gospel and wants nothing else. Some other leaders and pastors throughout the years display this same passion.
May we pray for glimpses of God's Kingdom like this. May it consume our hearts. Blaise Pascal, the mathematical genius, had a mystical vision in 1654, and he had his notes from it sewn into his jacket lining so he could refer to it, so completely did it change him. I encourage you to look it up... clearly something captured him. It starts out: FIRE. GOD of Abraham, GOD of Isaac, GOD of Jacob, not of the philosophers and of the learned. Certitude. Certitude. Feeling. Joy. Peace. GOD of Jesus Christ.
It goes on to mention tears of joy and a desire to never be separated from Christ. It concludes: Complete submission to Jesus Christ and to my director. Eternally in joy for a day’s exercise on the earth. May I not forget your words. Amen.