Reading only sources that already align with an existing view is comfortable, but it tends to produce a kind of quiet blind spot, a growing gap between what a person confidently believes and what a more complete, balanced picture would actually show if genuinely different perspectives were regularly included.
Deliberately reading a source or perspective that does not automatically confirm existing views, not to be immediately persuaded by it, but simply to understand its actual reasoning, tends to sharpen a person's own thinking considerably, if only by revealing which of their own assumptions had never actually been seriously tested.
This habit takes some discomfort to sustain, since encountering a genuinely different view is rarely as immediately satisfying as encountering agreement, but it is one of the more reliable ways to notice a blind spot before it quietly hardens into something considerably harder to correct later on. It is unglamorous work, which may be exactly why it holds up so well.