Number 5 Reason-The location
Monaco is centrally located in Europe and close to France and Italy. The airport is 30 minutes drive so you have easy access to the rest of the world. You are right on the ocean and the light from the Mediterranean is amazing-it makes everything sparkle. The beaches may be small but they're clean and not too hot.
Number 4 Reason-The weather
The weather is fantastic -I've lived in Australia, both coasts of the US and Hawaii and Monaco is much better weather-wise. The humidity is low and there is just enough rain to make things green. It's not too hot in the summer and not too cold in the winter but there are still seasons.
Number 3 Reason-The people
Monegasques are great people. Very friendly and genuine they have a unique culture which is very different from the French or Italian. They speak French, Italian, English and Monegasque language. Most citizens are working people and very entrepreneurial. There are also the people who were born in Monaco but are not citizens and they are very much the same. The expatriate community is diverse but mostly English and Italian (I don't count the French as expatriates).
Number 2 Reason-The social safety-net
The medical care in Monaco is outstanding and much better than anywhere else I have lived. Doctors spend time with you and get to know you as an individual. The hospital care is excellent and emergency services relaxed and uncrowded. The costs are low for services (about 1/10th the US) and are covered by insurance provided by your employer. Medications are also fully covered-but cheap even if you pay.
If you find yourself unemployed you are paid 60% of salary from 7 months up to 3 years (if you are over 50). The employment office actively assists you in job placement and you have priority.
The retirement age is still 60.
Number 1 Reason-The security
Monaco is one of the most secure places on earth. There are cameras and police everywhere and no one is mugged.
The police are very civic minded and helpful and polite to everyone and they speak at least three languages mostly four.
Public drunkenness and rowdiness are not tolerated and people are quickly swept up and asked to leave the Principality.