Cancun is almost in the center of the Caribbean and, unless there is a hurricane, enjoys bright sunny days and clear, beautiful nights year-round.
For Mexicans, Cancun is a honeymoon destination. For North Americans, it is a spring break destination. For Europeans, it is a holiday destination. Therefore, it is almost always crowded, but the weather is great all the time--that's why everybody is going there.
The entire Yucatan peninsula (including Cancun) has been a famous getaway for centuries. In the 20th century the island of Cancun was developed as a resort destination with world-class hotels and casinos.
You can still find peaceful solitude and pristine beaches in Cancun if you go at the right time. Spring is not that time because college kids love to come to Cancun for spring break, making for a wall-to-wall party. Summer is also bad because that is when Mexicans like to get married and Europeans like to take vacations. Fall is the most likely time to see rain in Cancun, therefore January and February are the absolute best months to see Cancun.
The benefits of a Cancun vacation are many: a chance to slow down and decompress, a great tan and memories of paradise to keep you warm until you can return again.
The weather in Cancun is so wonderful that it will change your standards for what constitutes a nice day. Weather elsewhere rarely measures up to a typical day in Cancun.