Imagine you own 20% of a 5-acre corn field (1 net acre). The majority owner decides to buy the adjacent 5 acres and does so by selling equity (of which you buy none). You now own 10% of a 10-acre corn field, so do you now own a smaller amount?
You own a smaller percentage of a bigger pie - your investment should have roughly the same value.
Of course, there are many other variables that determine whether you actually own the same total amount or not - why was the stock diluted? Does the diluting event increase the value of the company more or less than than the dilution effect (e.g. did the company acquire another company that increases net value through economies of scale, etc.) or is this a desperation move to stay in business?
But you didn't have anything "sold out from under you". At worst you had some voting power taken away, but very few people have enough voting power in companies to make that an issue. Plus if that is a deal-breaker you could always either buy more shares or sell the ones you have.