Unlike most of the other answers here, language itself doesn't really come into it; English is perceived by many people as the universal language because of the former influence of the British Empire and the current influence of American political and economic hegemony.

It is possible to try giving a strictly linguistic explanation; it could be that English is a simple language which is relatively easy to pick up. English has no noun genders, no complicated morphology, no tone system, it is written in the Roman alphabet which is pretty good at accurately mapping sounds to symbols, and the prevalence of English-language films, TV, and music makes it readily accessible and easy to practise. However, English also has an extensive vocabulary, a highly inconsistent spelling system, many irregular verbs, some problematic sounds such as "th", and a large inventory of vowels which can make it difficult for non-native speakers to tell words apart. Arguments about which languages are easy or difficult to learn are ultimately circular, as the perception of what is easy and what is difficult to learn depends on the person doing the learning. 

The second explanation is historical. The UK was the first industrialised nation, and discovered that one of the advantages to this was that they could colonise the rest of the developing world far faster than other European countries could. The British Empire covered a quarter of the globe at its largest, including North America, the Caribbean, Australia, New Zealand, much of West and Southern Africa, South Asia, and parts of South-East Asia. The UK set up English-speaking systems of government, industry, and exploitation in these areas, which established English as the language of global power in the industrial era. The British Empire finally fell apart after the Second World War, but the 20th century saw the transfer of power from one English-speaking expansionist power to another. The USA's cultural, economic, political, and military domination of the 20th and 21st centuries has ensured that English remains the most important and influential global language. As the official language of business, science, diplomacy, communications, and IT (not to mention the main language of the most popular websites), this is unlikely to change any time soon. 

Written by Gwilym Lockwood & Katrin Bangel