Expected value is the average outcome across many parallel trials. But you don't live in parallel trials — you live in one sequence, and in a sequence, hitting zero ends the game permanently. This gap is ruin risk, and it means a bet with positive expected value can be catastrophic if repeated with too much of your capital at stake.
The classic illustration: a coin flip that returns +50% on heads and −40% on tails has a positive expected value on paper. But bet your entire stack repeatedly and you go broke almost surely, because the multiplicative sequence of gains and losses compounds down, not up. This is why leverage destroys sophisticated people: it raises expected returns while introducing a path to zero. And zero is absorbing — you can't recover from it with a later good year. The practical rule: never risk what you can't afford to lose, no matter how good the odds look, because surviving is a precondition for compounding.