Not Jen

My name is Jennifer. You can call me Jennifer, Jenny or even Jenna, just not Jen. How come everywhere I go, people see the need to shorten my name? I always introduce myself as Jenny. I am already giving people a shorter version of my name, or a nickname, to use. Why do they feel the need to shorter it further? It's only takes a second to say the name Jenny, two little syllables. Often I feel the need to correct people. Other times, I wonder if it is worth it.