The brain is more interested in changes in sensory stimuli than signals that have been at a steady level for a long time. If you live near a busy dual carriageway/motorway for long enough you will grow accusomed to tyre roar and engine noise and, next time you take a break at a quiet hotel in the countryside you will find that you can't get to sleep because the noise isn't there any more.
Likewise, people with body odour problems usually don't realise it and everyone else is too polite to say anything, for fear of causing offence.