Paul wants the lives of the Philippians to reflect "glory and praise" to God. In order to do that, they must be able to discern what God desires from them and put it into practice.
The result of discernment in their lives will be that they will be (may be) "pure and blameless until the day of Christ" and enjoy "the fruit of righteousness that comes through Jesus Christ," which will "bring glory and praise to God."
Selfishness clouds discernment because a discerning individual is focused on the other person (their spouse in the example given in the lesson; God in Philippians 1:9-11). If we are selfish, it will be extremely difficult for us to discern what God wants from us (and, therefore, to please God). Discernment requires that we focus on God - and what He desires of us - not on ourselves.
We may be all about "good things" that are not what God wants from us. Those things may not be be bad, but they're the enemy of the best because they hinder us from being about "the best" (or what God desires of us).