They are not genetically related, but they have been in contact with each other for a long time. Japanese has borrowed lots of vocabulary that have been borrowe

They are not genetically related, but they have been in contact with each other for a long time. Japanese has borrowed lots of vocabulary that have been borrowed under regular sound patterns (depends when they borrowed it). Same goes for Korean and Vietnamese. Even if you are an illiterate Chinese speaker, you can make up Japanese vocabulary by converting Chinese words under these patterns, it may be hit or miss, but shared vocabulary makes it easier to learn. Add some basic grammar and you can start communicating pretty quickly. I know plenty of Chinese speakers that learned Japanese very quickly, but failed to learn English well. Other factors that make language learning quicker: access to materials (lots of Chinese speakers consume Japanese content, many classes available, there are a lot more books to learn Japanese for Chinese speakers), access to native speakers (close to Japanese, many exchanges students), friends and relatives that learned the language (if they can do it I can do it too), economic incentives, etc. Korean speakers also have an easy time with Japanese vs other languages even though they are not related. Shared Chinese vocabulary, simple phonology and similar grammar patterns.