Why Did My Son Have a Second Dose of MMR Vaccine?

My 10 year old son received a second dose of MMR vaccine today in Belgium, having received the first at 14 months in the UK.

I did not know children received a second dose so I looked up some information:
http://www.immunize.org/vis/mmr03.pdf - What you need to know about MMR
http://www.immunisation.nhs.uk/Vaccines/MMR/The_vaccine/mmr_two_doses Why are two doses of MMR necesary?

I know there is a huge debate about vaccinating children and the slim chances of a negative reaction. Personally, I think that I have to protect my children and others around them from even the smallest chance of contracting a serious or life-threatening illness. If the norm is to give these vaccinations to millions of children, that must be for a sound scientific reason.