Lfromj makes a great point about Britain's political stability, but personally I'd point to geography as a huge factor as well.
Britain was well-positioned in that it was culturally and economically a part of Europe and took part in the military, economic, and intellectual trends of the time, and yet as an island nation it was harder to invade and could place a greater emphasis on its navy than its competitors. Obviously, having a strong navy is a huge advantage in establishing and defending a far-flung empire, but so too is the natural protection from invasion. Contrast with the Netherlands and especially Portugal, whose colonial empires were negatively impacted by their having to contend with larger, more powerful neighbors.
On the flips side, Britain's general lack of continental ambitions after the 15th century served its colonial ambitions well. Although post-1689 Britain did have some German and Dutch rulers who were still concerned with the continent, and it certainly did get involved in some continental wars which took their toll, in general Britain could at least avoid the worst of the fighting in e.g. the Thirty Year's War and the Napoleonic Wars. These continental ambitions played a huge role in the decline of Spain (whose Habsburg rulers were constantly trying to defend their European empire and staunch the tide of Protestantism) and to a lesser extent France (who was largely stripped of its first colonial empire after the Seven Years War and Napoleonic Wars, but who, unlike Spain, was still powerful enough to gain new colonies in the 19th century).
A third major reason was economics, in particular Britain's status as the epicenter of the Industrial Revolution beginning in the mid-18th century. This answer is long enough already so perhaps I'll add to it later, but the tldr is that a lot of British expansionism was driven by its desire to control new territories to provide resources and markets for British manufactured goods. A great exploration of this in the 19th century is the book "
Empire of Cotton" by Sven Beckert.
