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

It seems the UK's Prime Minister Theresa May has decided to call a snap general election, despite having promised multiple times that she wouldn't, because she believes there is an overwhelming liklihood that this will keep the tories in power for longer - before a strong enough opposition could challenge them.

At the last election, the main parties were Labour, Lib Dems, and UKIP. However, as it stands at the moment, each of these parties have no real force behind them. Labour are in the worst state of unity they have been for quite some time with a leader, Jeremy Corbyn, which many within the party have tried to take down recently. UKIP have seriously quietened down since the last general election, and with article 50 having been triggered - their one main goal of leaving the EU is going ahead, leaving them with not much more to argue on. Then we have lib dems - they had a real growth in numbers of supporters, until they entered into a coalition government. Since then, they've never really managed to win back a huge potential audience - as they don't really trust them.

Now is the time for Tories to win even more of the electorates, because voters have lost faith in the other three at this weakened time.