I don't think I've ever told anyone without first being asked. Usually someone sees me checking my sugar, or injecting, or sees my pump and asks. Then I tell them the basics. As far as conveying the seriousness of having Type 1 Diabetes, I don't usually tell people. I've always believed showing is more persuasive and powerful than telling. The closer I get to people, the more they're around when diabetes is overtly impacting my life and so they gain a much better understanding of just how serious it is. The Drummer makes a good point, though. There are times when people, such as employers or possible employers, need to be told, to the point and up front. I was a Resident Assistant for a year and a half and I made sure to tell the Resident Director, my boss, that I might from time to time need certain accommodations. This way he didn't judge my performance unfairly.
I think in general it makes sense to divide people in to two different categories: those above you (teachers, administrators, employers, etc) and your equals (friends, family, acquaintances, etc). And you should have a different approach with each group as to how you tell them about your condition. It may sound a little calculating and diabolical but, honestly, expecting literally everyone to understand what you're dealing with is unrealistic and will exhaust you in every sense of the word. So when a person says something ignorant or malicious about your being diabetic, ask yourself, 'Who is this person to me?' If he or she is in the first category, you must stay calm and set the person the straight. If he or she is in the second category, try not to let it get to you as much. After all, there won't be the same repercussions as there might be in, say, the workforce, where you can face things like job discrimination. Just take a deep breath, and stick the person with a needle and tell them you just gave them diabetes lol