system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is gentrification and why is it so bad?
gpt
Back in the early to mid 1900s non-whites could only live in certain neighborhoods. So minorities bought houses where they could. Those homeowners (if they didn't move) may be anywhere from 70-90 years old.  As areas change and areas that people thought of as undesirable become more desirable, older houses get bought and significantly renovated or knocked down for larger more expensive houses. This causes values in the areas the rise, and property taxes in the area to begin to spike significantly. All of a sudden, the way the neighborhood used to look changes. What happened to all the people that used to live there?

 As those areas change, we start seeing the neighborhood lack the diversity that it once had and it has the appearance that those individuals who once owned in there we're pushed out. Often, this is why so many people think gentrification is bad. Studies have actually shown that gentrification has little to no effect on the decision of the  70-90 year old Grandma who lives in a gentrification neighborhood.

Now, where gentrification has its challenges is actually for renters. These are the individuals that are most likely effected by these neighborhood changes. These neighborhoods with lower property values tend to have a lot of landlords with renters. When the area gentrifies, rents that were now $800/may jump up to $1,000, then to, $1200, eventually $1500/mo, pushing out renters who work various jobs in the area. These are the people who nobody really cares about during the gentrification process. Once these neighborhoods have $500,000 houses in them, the community and investors stop thinking about the guy paying $800/month because they can make more money servicing wealthier clientele. 

Gentrification is a complicated issue. Taking a rundown house and making it nice isn't inherently bad. What we struggle with is, what should we do as a society to provide reasonable housing options for renters who get priced out of their neighborhoods?