I firmly believe that my internal world affects my external responses. If I practice meanness or self-doubt or generosity in my head, I are more likely to act that way towards other people and situations in the real world. This is why I stopped playing Age of Empires on my computers when my kids were young; somehow I found myself reaching for a head-lopping sword when dealing with toddler squabbles.
This matters with my reading as well. Sometimes I worry about the choices of the characters I'm reading; after all, since I read immersively, I'm spending time practicing whatever morality the author gives their characters; in the case of dark book or antiheroes that can teach me bad habits. This isn't really a worry, though, since I tend to engage and argue with characters as I read. But at a deeper level, the tone of a book affects my mood; reading a bunch of nihilistic literature at once depresses me.
This is one reason I like romance and mystery. Both are almost universally optimistic -- goodness will triumph, evil will fail. Mary Balogh's romance The Arrangement features a penniless woman and a blind gentleman, but I knew from the first page that they'd end up Happy Ever After. And from fairly early I figured out that this book belongs on the same shelf as Nora Roberts' Bride books -- there is almost no conflict. Occasionally the characters attempt to angst over a possible misunderstanding, but even they know that they are both decent people and cannot convince themselves to worry much, even if conventional books demand suspense. Instead it's a chance to spend relaxing time with two characters learning how to build their lives together, giving and receiving support as they establish their adult lives. Not the sort of thing I want to read all the time, but definitely a good choice when I want to reassure my subconscious that life can be good and people should be trusted.
On the other hand, reading Toni Morrison's Paradise is scarier; the women represented are strong, vibrant people, but the atmosphere is darker; things will not turn out well for almost everyone. Trust is not helpful. I think Morrison's book digs deeper into the reality of people; her viewpoint women convey much more complexity as they respond to big and small disasters and successes, but overall the weight of the sky waits to crush everything. I want to live my life as if I can trust the sky, so I think I'll make sure to sandwich each Paradise with several Arrangements. I like to stretch my mind and soul with books like Paradise, but I want to keep my instincts benevolent with books like The Arrangement.