When we read 2 Peter 1:5, 6, 7, we see that we are commanded to add and increase our self control, godliness, kindness and love. We are supposed to do all things in our life. It is our own effort. We must apply an effort to do those things.
But the Bible also says that we have the fruit of the Holy Spirit in us, like self control, kindness, goodness and love.
So it is not enough that we have the fruit of the Holy Spirit in us. It does not automatically make us live the Christian life, as God requires.
We also have our part to play. We are commanded from Jesus to love one another. We must put an effort to be patient, kind, gentle, loving our neighbour. It is not only about the fruit of the Holy Spirit in us. We have to decide to love others, and make the effort to be patient, gentle, kind, self controlled and merciful. We cooperate with God, who lives in us, and who helps us to love others, and to be patient, self controlled and kind.
In fact it says in verse 8, that those qualities in us must increase.
Yes, an unbeliever, who does not have the Holy Spirit, also can try to be kind, gentle, and love other people, but it is not the same. Because a person who does not have the Holy Spirit and his fruit in himself, although he tries to love others, he can not in the same way, as we as Christians can do, because we have the love of God, in our hearts, by the Holy Spirit, and an unbeliever does not.