No warranty if a laptop is dirty?

I have a dell laptop for two years, the warrant will expire next Aug. Today, it fail to boot. So i took it to a dell authorized repair person. He spent a few hours to check it up and told me that he found there are dust and debris(like hair) inside the laptop and the CPU fan did not work and that cause the motherboard and CPU overheating. The CPU and motherboard need to be replaced. The bad thing is that it will not be covered by warranty as he think the laptop has been misused. I think this is ridiculous.How can you expect a two years old laptop so clean without any dust and debris? I use it for normal work everyday.
Anyone could give me some suggestions?

Thanks.