Humans are very good at spotting patterns in random sequences. The seventh wave is always the biggest; Friday the 13th is always unlucky; sudden lulls in the conversation always occur on the hour or at 20 minutes past or 20 minutes to. These patterns seem to appear partly because we generally have fairly loose criteria for what constitutes a special pattern and partly because we are very good at excusing or forgetting situations where the pattern is broken.
It’s said that hay fever follows a seven-year cycle, but seven years is a long period in a human life. If you’re 35 now, you only have five data points on your graph, which isn’t enough to draw meaningful conclusions. Hay fever will be better or worse from year to year according to the climate, where you live, how much time you spend outside and lots of other variables. All of these, scrambled up with all the things that affect all the other allergies, form a random sequence and any pattern you think you see is mainly wishful thinking.