It goes hand in hand with an increased cost of living (e.g. the nicer things get the more expensive it is to live there) which in turn causes a displacement of people and communities who lived there before the gentrification happened. In many western countries there is also a strong correlation between the above displacement and it overly effecting minorities, who due to economic/political/social disenfranchisement have congregated communities in low cost areas.

If you are a member of the more affluent population who moves into a gentrified area, it's great, but if you're a part of the community who has lived there for a long time and you suddenly can't afford to any more, it's really not great at all (to it mildly).

So in a narrow sense 'gentrification' isn't a bad thing, however put in the wider context it can have very significant negative effects on poorer communities. Especially when combined with a lack of social/affordable housing.