I've been told I'm hard to read. Once I was on a 6 week jury and after we rendered the verdict, lawyers for both sides approached me to ask my opinions on the case because I was so inscrutable. In other words, I play my cards close to the vest.
Sometimes, while I'm keeping to myself or "just minding my own business," I'm really closing myself off from people. While I'm observing, I'm calculating risk. Is this person friend or foe? Are they safe to open up to? Do they need help or are they offering it? I guard my heart.
In certain circles, believers (especially singles) are admonished to "guard their hearts" when it comes to relationships. This presumably comes from Proverbs 4:23, which says, "Above all else, guard your heart, for everything you do flows from it." But Proverbs 4 isn't talking about relationships per se, it's talking about wickedness, violence, perversity and corruption.
We are to steer clear of evil enticements so that evil won't, in turn, flow out of our own hearts. It does not say to steer clear of people so we don't get hurt. In fact, as Christians (aka Christ-followers), we are to do just the opposite. Christ knowingly and intentionally spent time with Judas who would betray him, and Peter before and after he denied knowing Him.
Jesus didn't look for ways to buffer Himself from others. He waded in. He welcomed and accepted. He was (and is) a friend to the friendless- even those who are sketchy to be around. If I am going to model myself after Jesus, I can't hide behind a twist of scripture to validate guarding my heart from others.
Jesus was (and is) hard-core. He runs into fire to rescue us, without giving thought to Himself. He draws us near even if it means rejection, betrayal, apathy. He is forever putting Himself out there, turning the other cheek and asking us if we're willing to do the same.