As for me it was a the love that God placed into me at salvation. I know my story is a little different but I was a severe alcoholic with panic and anxiety. [This because I was miserable with fibromyalgia and arthritis]
I finally hit rock bottom, No money, No nothing, So I hit my knees and cried out to God to kill me or help me.
From the moment I ask Him, He delivered me of alcohol, Smoking, Dipping and Panic immediately. I never wanted or desired any of it again. All those years of drunkenness just washed and forgotten.
God then filled me with a great love for others at that moment and I guess I felt the calling at that time but was pretty doubtful about my ability as the life I had lived was pretty bad.
I started going to church and reading and studying the Word of God day and night. I started seminary school and made the decision to serve God the rest of my life somehow and so I prayed for an answer. The very next day a man came up to me and ask if I would be interested in going to the jail and giving my testimony. Well one thing led to another and I became ordained and started volunteering preaching at jails, prisons and nursing homes around my area. Then some people approached me about starting a church because of various reasons that their home churches had started taking a "softer" approach on things that God declares to be sin. So I prayed on this a long time and finally got my answer.
Now I am a volunteer pastor for our very small but loving church. I do not pass a collection plate at all, Nor will I ever, there is a little box on a desk that people can drop money into out of a cheerful heart "if" they wish, or as they might feel led. I say this simply because I want to make a point.
God always provides. Our little church never has much, but it always has just enough, He always makes sure that if someone comes to us needing diapers or food or shoes or medicine or whatever, There is always "just" enough to cover it.
You see, it is God that blesses this, So He will your decision as well. if you ask for His complete guidance. I always tell others, it is about true faith and if you believe then anything is possible :]
If you feel "called" then pray and let God have you completely . He needs laborers and He can and will use you greatly.
God bless you in your journey and I pray that you lead many souls to the Lord :]