Is there a reason why Leopold I was chosen to be the first king? The wikipedia article on the Monarchy of Belgium says that the National Congress on 1830 originally nominated the son of Louis-Phillipe of France to be their King.
The National Congress of 1830 was seriously considering only 3 candidates: Louis, Duke of Nemours (the son of King Louis-Philippe of France), Auguste de Beauharnais, 2nd Duke of Leuchtenberg and Prince Leopold of Saxe-Coburg (who eventually became King Leopold I).
The congress was heavily under the British influence. Louis, Duke of Nemours was not an acceptable figure for the British, who were afraid that the new Kingdom would be too heavily influenced by the French, if the son of the French King became its Monarch.
There were also other reasons why the Duke of Nemours wasn't chosen King of Belgians; he accompanied the French army that entered the Belgium to support it in its fight for independence from the Kingdom of the Netherlands, and took part in the siege of Antwerp. Therefore, he was unacceptable candidate for the Netherlands as well.
King Louis-Philippe personally declined the honour of accepting the Throne for his son (even though the final decision of inviting the Duke of Nemours hadn't been made at the time, so it was a symbolic gesture).
Auguste de Beauharnais, 2nd Duke of Leuchtenberg was considered too young to become a King for the newly founded Kingdom, which needed an able ruler to resist possible attack from the Netherlands (incidentally, soon after Prince Leopold of Saxe-Coburg became King Leopold I, the Netherlands did attack the newly founded Kingdom of Belgium). There were also reservations concerning his health (not unfounded, for he died at the age of 25, 5 years after the National Congress). His candidacy was supported only by the Dutch, but actively opposed to by the British and French.
Leopold was almost a perfect candidate for most of the parties involved (except France, who was initially apposed to his nomination).
Britain actively supported Leopold, as he was the widowed of Princess Charlotte Augusta of Wales (the only daughter of King George IV). King George (then Prince Regent) was very fond of his son-in-law even after Charlotte's death; a year after Princess Charlotte's death, the Prince Regent granted Prince Leopold the style of Royal Highness. Although King George IV died months before the Congress commenced, his successor, King William IV, was also very fond of the husband of his late niece and actively supported Prince Leopold's candidacy (especially since the other two were the unacceptable (for British) candidates put forward by France and the Netherlands - Duke of Nemours and Auguste de Beauharnais).
Russia was also strongly for Prince Leopold's candidacy; at the age of 5, Leopold was appointed Colonel of the Izmaulovsky Imperial Regiment in Russia, at the age of 13 he was already a Major General. Although Prince Leopold was initially inclined to join Napoleon's army, he changed his mind after Napoleon's invasion of his native Sage-Coburg and continued his career in the Russian Imperial Army. He successfully campaigned against Napoleon during the latter Russian Campaign and won praises for his courage and leadership in the Battle of Kulm. At the age of 25, Prince Leopold reached the rank of Lieutenant General in the Imperial Russian Army.
He was exactly the sort of King/Commander the Belgians needed at those turbulent times. He enjoyed the support of Britain and Russia and no major party opposed him. Even France withdraw its objections soon after the Congress's start (possibly after his engagement to Marie-Louise of France, the eldest daughter of King Louis-Philippe of France, was agreed on). With the backing of Britain, Russia and France, it was hardly surprising he was eventually chosen to become King.
There was one more candidate, which was briefly considered by the congress - Archduke Charles, Duke of Teschen. However, the Duke of Teschen never had a lot of support and none of the major parties (apart from the Dutch, and only briefly) considered him seriously.
