Reluctance means "unwillingness." Your reluctance to travel on an airplane would be understandable if you glimpsed the pilot and copilot wearing party hats and blowing noisemakers.
If you'd rather not do something, you feel a sense of reluctance about it. You pause for a moment, maybe to consider whether you should really go through with it, or just to enjoy the last moment before it happens. The word comes from the Latin reluctari, or to struggle against. There's always a struggle when reluctance is present, in that you feel pressure to do something or in not agreeing right away, someone is going to be disappointed.