I think they are several factors .Firstly ,the mother language ,if the language that you want to learn is near to your mother language , you can learn it easily.For example ,my mother language is Arabic, so can learn Urdu and Farsi easily. The second thing is that, how can you think about the language that you want to learn ,when I was chatting with my Chinese friend I asked him about his language ,he said It is easier than Arabic ,before that I thank it is difficult to learn Chinese language .