Why do some defect?

But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be found out. Since all these things are to be destroyed in this way, what sort of people ought you to be in holy conduct and godliness, looking for and hastening the coming of the day of God, because of which the heavens burning will be destroyed, and the elements will melt with intense heat! But according to His promise we are looking for new heavens and a new earth, in which righteousness dwells. Therefore, beloved, since you are looking for these things, be diligent to be found by Him in peace, spotless and blameless, and consider the patience of our Lord as salvation, just as also our beloved brother Paul, according to the wisdom given him, wrote to you, as also in all his letters, speaking in them of these things, in which are some things hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction. You therefore, beloved, knowing this beforehand, be on your guard lest you, having been carried away by the error of unprincipled men, fall from your own steadfastness, but grow in the grace and knowledge of our Lord and Savior Jesus Christ. To Him be the glory, both now and to the day of eternity. Amen. - 2 Peter 3:10-18 (LSB)

Scripture clearly tells us that there will always be defectors. And as you look in the gospels, for example, in the New Testament, you’re going to see some of the reasons why.
Ignorance is one reason why people defect from the gospel. There’s never really any penetrating of the truth, that’s the hard ground. It just sort of bounces off them. They don’t comprehend it.
Persecution is another reason. People who are exposed to the gospel but do not want to pay the price of naming the name of Christ and bearing the reproach of Christ.
“The worries of this life,” Jesus said, “are another reason why the seed has a very short life and produces no fruit.”
And Jesus also said, “The deceitfulness of riches.” People love wealth and materialism and prosperity in this life more than heavenly promises.
The teaching of Jesus is hard. John 6:60, “Many, therefore, of His disciples, when they heard this said, ‘This is a hard statement; who can listen to it?’” A few verses later, “As a result of this, many of His disciples withdrew and were not walking with Him anymore.”
Another reason is Satan’s devices. Paul says he’s very concerned that people will be led away from the simplicity of devotion to Christ. Satan will come in and confuse people as to the priority and the person of Christ.
There are some people who defect from the gospel because they don’t want to be alienated from the people around them. They loved the world too much. And John said, “If you love the world, the love of the Father is not in you.” - John MacArthur

What should my response be?

Therefore I exhort you, brothers, by the mercies of God, to present your bodies as a sacrifice—living, holy, and pleasing to God, which is your spiritual service of worship. And do not be conformed to this world, but be transformed by the renewing of your mind, so that you may approve what the will of God is, that which is good and pleasing and perfect. - Romans 12:1-2 (LSB)

Test and evaluate yourselves to see whether you are in the faith and living your lives as [committed] believers. Examine yourselves [not me]! Or do you not recognize this about yourselves [by an ongoing experience] that Jesus Christ is in you—unless indeed you fail the test and are rejected as counterfeit? - 2 Corinthians 13:5 (Amplified Bible)

Be Encouraged to present yourself as a living sacrifice to your Lord and Savior Jesus Christ.