In Chapters 32 we see two and half tribes deciding to opt out of the Promised Land for what they determined to be greener pastures. It seems that they were originally also attempting to opt out of the coming conflict, but they exhibited some spiritual maturity when they capitulated to Moses' argument that they could not and should not. To at least some degree, they gave in to worldly temptation and unbelief by choosing these greener pastures over the Promised Land God intended for them.
An argument could also be made that God's plan was flexible and He allowed the Promised Land to be enlarged as long as it was done the right way.
What is likely key in this passage is Moses' argument and how he persuaded them not to sin. Moses' first point is that the sin of one can negatively affect the faithfulness of others. Your sin does not only affect you. Moses did not want their disinterest in the Promised Land from discouraging the other Israelites. His second point was that if they chose to sin, God would not allow that sin to go unnoticed or unpunished.
Both those points are true for us as well. Our sin affects those around us. And, even though the sins of believers are forgiven through the blood of Jesus, we are not absolved of the consequences of our sin. The Bible gives example after example of this - the death of David's son due to his sin of adultery and murder is just one such example. God will not be mocked, and we do reap what we sow, and grace is not a license to sin.
There are many reasons to obey God. Chief of which should be because we love Him and are grateful for His redemptive work in our lives. But other reasons include the fact that our sin will always be brought to light, we cannot escape earthly consequences for our sins, others depend on our faithfulness, and others are damaged by our faithlessness.
In our Matthew passage, what struck me (and ties in well with the theme from Numbers 32) was the temptation of Jesus. Even though Satan tried to twist the very words of God to entice Him to sin, Jesus did not take the easy way out. He obeyed His Father out of love, and gave us an example in His faithfulness.
Tomorrow's Bible In a Year Passage passage: Numbers 35-36; Matthew 5:1-26