Paul is reminding his friends in Thessalonica of his ministry attributes and habits. It's been awhile since he visited, and he wants to jog their memory concerning his work amongst them. Lest they reinterpret his motives and actions, Paul makes a point to reassert his authority and his love for them. Central to this reminder is the authority over him, and the love that motivates him: God and His Gospel.
I'm sure that Paul's maxim for speaking got him in trouble often; it would get lots of people fired if they applied this rule at work. What if you spoke the truth - even in love - to your boss concerning what you really thought about their management style? What if you spoke plainly to the fasttalking cubicle mates, avoiding all over-generalizations and thus being a lousy conversationalist? But then, speaking to please God and not man may get you a raise, since you'll be more trustworthy, more reliable, more loving, more encouraging, and more redemptive in your work and words.
For all of Paul's faults, he worked hard to speak only according to the Gospel. He wasn't out to make up new truth, but rather to proclaim it, and encourage others to live by it.
Petra has a great song from their 1982 album "Not of this World" called "Godpleaser". "I want to be a Godpleaser, don't want to be a manpleaser". I've grown up with this song, and thus this truth, in my ears and head for a long time. Doesn't mean I live by it as often as I should. But I do remind myself often, since I end up speaking alot - maybe too much (pastors can be prone to verbosity), to speak to please God first, and through that men and women may or not be pleased.
This maxim partly explains the instructions he writes to the church for how to conduct themselves: admonish the idle, encourage the fainthearted, help the weak, be patient with them all. See that no one repays anyone evil for evil, but always seek to do good to one another and to everyone. Rejoice always, pray without ceasing, give thanks in all circumstances for this is the will of God in Christ Jesus for you.
Think about how counter-cultural, how subversive that list of commands could be for you at work and school, even at home or church. To silently, but steadly speak only to please God; this would unnerve those grasping to project a particular image (which is rooted in pleasing people, hence in opposition to your strategy), and this would irritate those seeking to assert power in order to accumulate unjust gain (pleasing themselves at the expense of Imago Dei's).
Oh how easy it is to speak and serve in order to please others - in order that they may be pleased with me. It's a trap, to live dependent upon the fickle pleasure of others. It can be a trial seeking to please God, since he is invisible and incomprehensible. Yet, like Paul, faith in God propels me to please Him first, to seek his coming kingdom first, to desire his righteousness first, to delight in him first. And he'll take care of all the rest.