It's easier to learn a language that shares more in common with a language you already know. This includes vocabulary, grammar rules, and phonology. For example, English borrows a wealth of words from French and Latin, which makes picking up the vocabulary faster. English and French also have both become less inflected and more isolating over time (that is, word order and linking words like prepositions and auxiliary verbs do more to determine syntax than various inflections of verbs, nouns, pronouns, and adjectives do).Mandarin Chinese is an isolating language like English (even more so, actually); but the writing system, lexicon, and phonology are all grossly different.

Other factors are motivational: psychological and sociological. For example, you may be more interested in learning Russian because of romantic notions of old Czarist Russia or the Soviet Union, or you may simply enjoy Russian literature. On the other hand, an immigrant coming to the United States experiences considerable sociological pressure to learn English (employment, fewer xenophobic responses, etc.).