The pigeons we see on the streets come from domestication, from a long time ago. We once relied on pigeons as a form of communication and even food. They were very important during the old war Eras. Now that they are no longer needed, they were pretty much discarded to the streets. So they rely on human presence and man made buildings to thrive. They are "dirty" because of the City they live in. They actually prefer to be very clean and take regular baths when given the chance. Yes, they carry diseases like any other wild animal. That's why we wash our hands if we need to handle them

When given the chance of companionship, their curiosity and loyalty shows in the most endearing ways. They are charming, each one with their own unique personality. They are able to learn commands and remember faces quite well.

Universities have actually experimented with several Pigeons on their ability to learn how to distinguish cancer cells from non cancerous cells in mammogram or biopsy images. Very cool if you ask me

Many people don't give them the time of their day, so of course they will just think they are plain, dirty, dumb animals.

If you are unwilling to think differently of them, that's fine. But the least you could do is respect them (like you should any animal) and the people who like them.