You wouldn't approach a prospective connection in person as informally as you do online, would you? If you were meeting prospective clients at a networking happy hour, you wouldn't approach them with an automated greeting, would you? If you were pursuing a business lead and met them for lunch, you wouldn't spit out automated messages throughout your meal, would you?
It's the same online. You would start with a warm greeting and build on valuable conversations that manufacture personability and importance. Through LinkedIn's personal messaging, you can find a much greater return through an increase in network connections, the possibility to create new business relationships and increase sales.
If you are connecting with someone, there should be an obvious rhyme or reason, such as a shared industry, shared connections, or shared location. Sharing something simply means you have something in common, and if you have something in common, then you have something to write about in your personal message.
