There's more to developing allergies than heredity. What you become allergic to is based on when and how much you're exposed to a substance and how much of it you're exposed to.
For example, say you have a tendency to be allergic to mold spores. You may have no allergy symptoms when you're living alone in your spic 'n' span apartment, but when a roommate moves in, bringing along a jungle of houseplants, an old mattress, and a humidifier (to keep her skin moist), you soon become a symphony of sneezes and snorts. What happened? You had endured a certain amount of exposure to mold spores without a problem, but once the scales were tipped by the onslaught of your roommate's mold-bearing stuff, your immune system kicked into high gear.
How old you are when you're exposed is critical. Recent studies show that heavy exposure early in life -- before 2 years of age -- may be protective against animal allergies and asthma.
Where you live may also affect the degree to which you suffer from allergies. Say you're allergic to the big four: grass, pollen, dust mites, and mold. If you dwell in the Pacific Northwest, where all four are abundant nearly year-round, you may suffer a lot of the time with chronic allergies. Your nose will drip, you'll sniffle, and you'll have a sore throat from postnasal drip, but your symptoms won't be extreme, just ever-present. Move to a higher and drier region, where the grass grows wild but mold spores and dust mites are less common, and your allergies may become seasonally acute (sudden and severe). You might find yourself sneezing uncontrollably for a month but then your symptoms will abate. Living with allergies is often a game of give and take, especially if you suffer from several kinds.
For more information, see the links below.