I can almost cry whenever I want to, its actually a pretty good skill, for an actor anyways. It helps if you are deep, in touch with your emotions, care about stuff, and are able to see and reflect on the all the sorrow out there. It's not the desire to cry that deserves further study, most likely it is a fear that one is becoming cold, heartless, indifferent, and inhuman.
I know what you mean though. When my dad shot himself when I was 16, I wanted to cry, but I couldn't. I was upset that I could not cry and wondered at how heartless I seemed to myself. I understood in my mind that this was the kind of wound that started off soft and would only get worse over time.
Something that may help, and some songs that might do the trick:
Imagine you are completely trapped and forsaken inside of a strange place, or haunted museum. Left there over the weekend, locked inside somehow. There is no food, but I'll give you a water fountain and some bathrooms. It's going to be a dark and lonely weekend. The second one is from THE album that activated my dreamy deeper side back when I was thirteen.
[youtube]66mghzJCY2s[/youtube][youtube]jwAop6MVxa4[/youtube]
[youtube]1gMxKBLftq4&ob=av2n[/youtube][youtube]7irynJtKI4I[/youtube]
Not sure why I thought of this one, but it is a sad song. This last one is a topic that may also be of some interest. Raising your vibrational frequency. High vibration, is not something I can explain. It sort of feels like a powered up super saiyan transformation, only entirely emotional and spiritual. I'm not buying the whole ascension thing, but at least it had a nice song and video instead of a bunch of talking.