OK... so only you can answer the question! But it is useful to find out what other people think about the subject. Your real reasons might seem trivial (e.g. "I thought the course would be easy and give me more time outside and not so much time inside with books") - but the real personal reasons are what drive motivations, and can have more than just personal significance. Many scientists do not really explain their personal reasons for doing research, or entering a particular field of study, and this is a pity, because it can help to communicate a subject if the writer is able to link it to the same kinds of ordinary confusion and concern that most people face, in all areas of life. Don't be shy about giving your real reasons, but show that you are conscious of other reasons that others might choose as explanation, even if they are not your reasons. How does anyone make decisions about anything, in reality? The subject of decision making is an entire research field in its own right.... and has applications throughout the sciences. So - speak from your heart, but be analytical at the same time.
Peter
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Peter J. Matthews, Chief Admin.,
The Research Cooperative,
Auckland & Kyoto.
Contact: researchcooperative [at] gmail [dot] com