Why Must You Guard Your Health?

When there's a possibility of peril and you aren't careful, you're not guarding your health or your safety,  you have to know it's a sin. And don't think it's a small sin. It's all included in the sin of shedding blood. You'll say, "Well it's my own blood." No, it's not your own. It all belongs to Hashem. Nothing is yours. If you shed your own blood Hashem will make you pay for it. Oh yes. -- Test of Emunah (#557)