Why should we compare Scripture with Scripture when studying the Bible?
Comparing Scripture with Scripture is the best way to study because no Scripture is a private interpretation. Peter writes that doctrine cannot be built on isolated verses here and there; doing so can build anything. Using all Scripture from Genesis through Revelation ensures the whole fits together, and seeming contradictions fade away.
Les explains that the best way to study is to compare Scripture with Scripture, because Peter says in his little epistle that no Scripture is a private interpretation. This means you cannot build a doctrine on one verse here and one verse there, because you can build anything when you do that. Instead, it is our prerogative to use all the Scripture from Genesis through Revelation and see that it all fits. Seemingly contradictions may arise, but when you study they just sort of fade away and are not contradictory at all.
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Why should we compare Scripture with Scripture?
You should compare Scripture with Scripture so that you don't claim a passage doesn't really mean what it says; if you don't believe what it says in one place, find another place that corroborates it, and it all fits.