I think I can understand the feeling of having a "calling" for it. For myself, personally, I've always felt pretty drawn to nursing. However, I was so afraid of failing, I continuously made decisions that led me away from it. It seemed to me that every time I changed my major away from nursing, something would push me back (i.e. the new plan would fall through, it wasn't a good fit, etc.). I've personally always felt an intense admiration for nurses. The summer after I got my B.A. degree, I volunteered for habitat for humanity. While on the job, temperatures spiked high and I ended up with heat exhaustion. Fortunately for me there were both a doctor and nurse there that day. Initially both checked on me. Then, the doctor went about his way while the nurse sat and talked to me for the better part of that afternoon (taking care of me at the same time). It turned out that she was also a nursing school instructor. She had inspired me to go for it. Well, I went back for the last two pre-reqs I needed (microbiology and nutrition). Then, for reasons I question today, I decided once again to do something different (truth is, i was running from nursing). I joined an alternative teaching program. I thought it was an easy way out. Boy was I wrong. I failed. I failed hard.
Ever since I made the decision to get over my fear and apply (and be accepted) for nursing school, everything in my life just seems to be falling into place. I'm not usually a very religious person but at a certain point you have to wonder how much of it is coincidence and how much of it is someone smacking you in the face over and over until you finally comprehend what you're supposed to be doing.
I believe in my heart that this is what I am supposed to be doing and I guess some might say I feel a "calling" to it. Seven years of major changes and I always come back to nursing. After a while I had to stop putting it onto a pedestal and just go for it.
Er.. I went off on a bit of a tangent. I guess what I mean to say is that feeling a "calling" really depends on how you read the situation and based on your definition of a calling. Perception is individual based.