What has motivated the UK's Prime Minister Theresa May to call a snap general election?

Commentators in the UK have been suggesting for some time, since Theresa May became leader of the conservative party in te wake of the EU referendum results, that the Conservatives may call an early election. The Labour party, the next biggest party in parliament, have been struggling after disappointing results in the 2015 General Election and the election of a new and more left wing leader, Jeremy Corbyn, in September of that year. The Conservative party experienced an unexpected opinion poll boost after their leader David Cameron resigned (having failed to secure a yes vote for remaining in the EU) and was replaced by Theresa May. lthough May campaigned against Brexit, she was careful not to be too associated with the Remain cause. As such, she is seen as a much more pro-Brexit Prime Minister. As the Conservatives have risen in the polls, the largest pro-Brexit party - the UK Independence Party, or UKIP - have seen a substantial poll rating fall.
Meanwhile, the Labour Party are badly split, many MPs being opposed to the more radically left wing party leader, who is popular with grass roots supporters but unpopular with most swing voters.
Given all this many commentators see the election as pure opportunism, Theresa May taking advantage of a poll lead, based partly on the popularity of Brexit, in order to seure a large majority for the Conservative party in parliament.
However, it's also true that Britain will be leaving the EU in 2019, a year before the next scheduled General Election. If anything were to happen to reduce the Conservative's slim majority in pariament, the prime inister might struggle to pass legislation needed and has claimed this would hamper her negotiations of the final stages of Brexit, particularly as some parties, such as the liberal democrats, have argued that they woud obstruct any move by the UK to leave the European union. Securing a large majority for the conservatives now woud make such obstruton all but impossible.