Why exactly do Europeans hate Roma again?
Well if you spend any amount of time in Europe, or pay attention to the domestic news of European countries, they feature a lot in petty crime. Almost everywhere in Europe they have a reputation for pickpocketing, theft, burglary, etc. However, I don't think this is enough to explain why they are especially disliked; after all, many other ethnic groups engage in the same crimes, but don't get the same level of hate.
The deeper resentment comes from them being an ethnic group with origins outside of Europe (India), who are living in Europe but in many--the majority--of cases, are refusing to assimilate to the identity of their respective European nation. By this I mean, yeah they might go to work and school like everyone else, but they still retain their own language and especially their own culture. For an American, or Canadian, being from nations founded on immigration, it might be hard to understand why this angers people in Europe. The problem comes from how prominent nationalism and ethnicity feature in the way many European people see themselves, especially those in the more ethnically homogeneous countries. Therefore, while the American says "why does it matter, they should be allowed to celebrate their own culture", the European says "my people have been here for thousands of years, if you want to live here too then adapt to our culture". The Roma of course, refuse to abandon their culture, so they remain perpetual outsiders in their nations.
Of course, these are the "domestic" Roma. Many Eastern European nations have large Roma populations. You also have the ubiquitous anti-immigrant reaction coming in when you look at Roma populations in Southern and Western Europe, as many immigrated from Eastern Europe after their countries joined the EU.