Quick answer
God's attention reaches the level of thought, not just conduct. What is planned in private is as visible to Him as what is said out loud.
Read both lines
This is the third "abomination to the LORD" in the chapter, after the wicked man's sacrifice (15:8) and his way (15:9), and the target has moved steadily inward: offering, then road, then thoughts. Placed after 15:11, the progression makes sense, since a God to whom the human heart lies open is not evaluating the surface. The second line answers with pleasantness rather than righteousness, which is a gentler and more social word: the words of the pure are agreeable, good to be around. What starts clean at the center comes out pleasant at the mouth, the same route this chapter has traced at verses 2, 4, 7, and 28.
Related: Proverbs 15:8 · Proverbs 15:11 · Proverbs 15:28
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Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.