"Now may our God and Father Himself and Jesus our Lord direct our way to you; and may the Lord cause you to increase and abound in love for one another, and for all people, just as we also do for you; so that He may establish your hearts without blame in holiness before our God and Father at the coming of our Lord Jesus with all His saints." (1st Thessalonians 3:11-13)Of first note in this sentence is the term "saint". This is the same Greek word often translated as "holy" as in the Holy Spirit. We could just as easily say that we are His "holy ones" and He is His "Saintly Spirit". It is also important to note that when Paul refers to the "saints of God" he is referring to all believers, not just special people who have been so designated by the church ecclesia. Paul consistently, when using the term saints, refers to the entire body of believers as does Jude and the writer of Hebrews in their usage of the same term. Peter uses this same word in "holy priesthood" (1st Peter 2:5) and "holy nation" (1st Peter 2:9) in describing the catholic collection of believers. We are all saints before God; saints through of our redemption and sanctification by Jesus Christ.
Secondly, Paul prays that God would "cause you to increase and abound." He does not pray that God would increase them, but rather that He would cause them to increase. Character and the fruit of the Spirit are not something that God grants through prayer as a completed gift in to our lives, rather, they are developed, or grown, in us throughout our lives as we learn to walk with and trust in Jesus. Character and fruit come when we learn to express our faith through actions of love. For example, God says we should love our wives. We believe this and look for ways to express God's love to our wives and, as such, over time, we become more loving. God says that the tongue holds the power of life and death. We believe this so we learn to practice the wisdom of God in all our speech and conduct and, as such, over time, we develop a manor of speech that brings life, not death, to people. Similarly, God uses situations and circumstances as opportunities for us to grow in our faith and trust in God. When my wife and I were first married we had little money, yet we saw regularly how God miraculously provided for our needs, even our rent each month. This season of our life taught us how to trust God with our finances, a lesson that has yield great dividends over the nearly thirty years of our marriage. Let us always remember: some things God gives and some things He causes.
Finally, the reason Paul prayed that God would cause them to increase and abound was that they would progress towards holiness and that they might be able to stand blameless before Him at His return. The key to their development in holiness was their increase in love; love for God, love for one another, and love for all people. Love is not a feeling, it is not an emotion, love is a decision that leads to an action. Consider one of the most familiar verses in all the scriptures. "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life." (John 3:16) God loved us, not just with a feeling, but with a choice (to redeem us) that lead to an action (sending His Son to die in our place). Singular, When Paul describes love, he describes it as: "Love is patient, love is kind and is not jealous; love does not brag and is not arrogant, does not act unbecomingly; it does not seek its own, is not provoked, does not take into account a wrong suffered, does not rejoice in unrighteousness, but rejoices with the truth; bears all things, believes all things, hopes all things, endures all things." (1 Corinthians 13:4-7) These are all actions. Paul prayed, not that they would increase in loving feelings towards one another, but that they would act more loving towards one another. It is the increase in our actions, representing faith working through love, that is the key to growing in holiness.