Psalm 78:70 says that God "chose David...from the sheepfolds."
David was the youngest of Jesse's eight sons. With red hair and apparently a handsome kid (see 1 Samuel 16:12), he tended the sheep on his dad's pasture. The strength and courage needed to kill Goliath had been developed through the years protecting them from lions and tigers and bears. Oh my. Okay, not really tigers, but he does say he killed the other two in protecting his flock (see 1 Samuel 16:35; the story of David and Goliath takes place in chapter 17). Jesus does that. He says He'll leave the "ninety and nine" to go after the one stray, "lost sheep" (Luke 15:4).
Life doesn't necessarily contain many life or death moments—that is unless you're a soldier (or a Marine) or police officer or firefighter. At least not in our "civilized" western world. Making these hard decisions is far less frequent than, say, in feudal Japan under a shogunate (where slight mistakes are atoned for by ritual and assisted suicide) or among the Yanomamo tribe of South America. The Yanomamo are excruciatingly violent toward, not just neighboring tribes, but also their own members. Point is, violent force is not necessary in today's world to get our point across. In defending our loved ones however, I believe it can be necessary. With Goliath however, diplomacy was already out of the question. The other Israelites were content to lie down and let the Philistines encroach. But I believe the real reason David decided to go and fight Goliath was because he "def[ied] the armies of the living God" (1 Samuel 17:26). I'm reminded of "Beat It" by Michael Jackson. David's the exception to that song. The rest is history. And now we have the perfect metaphor for dealing with the problems of our own lives that not only seem, seem huge and insurmountable, steadily creeping and seeping into our life, but that no one else wants to face. Goliath stood on the other side of the battlefield and taunted the Israelites. Empty threats as far as David was concerned.
Before I go any further, if you're having an issue with this story, as far as its solution (i.e. the slaying and decapitation of Goliath, 17:51) may I plead the rule of cultural relativism? One corollary of cultural relativism says that it's wrong to judge other cultures with the morality of our own. That may have pulled the story from the sea of subjectivity only to have it cast again into the deep end, but it was indeed a different time and place.
Consider this verse from Isaiah: (40:11) "He (God) shall feed his flock like a shepherd: he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young."
The pastoral vocation was common in the Middle East during that time. Still is. David tended sheep because he was the youngest. Probably because no one else wanted to do it, too. This verse in Isaiah describes the character of God as kind, gentle, caring, concerned. God is like this all the time. Even when we don't feel it. Even when we think that He's just the opposite or not even real at all. One question I have for you is, with the unique spiritual problems you're facing, do you think God might choose you to be the one to deal with them, once-and-for-all? If so, have at it! If we have the courage to believe that God is as this verse describes—even when the devil shouts at us and fear and shame and discouragement begin to press in—then we are believing what is true. And God will prove Himself to be the best that we believe. Conversely, if we choose not to believe, God isn't necessarily obligated to reveal Himself to you. If you know anyone like that, pray for 'em.
So, what are the huge crises that are worrying you? (financial, physical, familial...?) This takes real effort, but choose, will to believe that God is bigger and that He has a way out of this suffering. Paul said as much. God "will with the temptation also make a way to escape, that ye may be able to bear it" (1 Corinthians 10:13). He was referring to temptation but don't you think David was tempted to run from Goliath? Maybe not. Maybe through the years of his shepherding routine, he developed the resolve that stood up and in the face of the audacity of Goliath. Resolve to remain faithful through the slow downtimes of your life and you'll be slaying giants before you know it.
Jesus says to "say unto this mountain…" (Mark 11:23). See what happens in verse 24.
God bless you!