I have written business plans for all manner of industries: a coin operated jukebox company, airlines, travel companies, new product launches, and anti_aging product companies. It is not necessary to have a passion for the product or the company to write or develop a business plan. What you must have is a passion for aggregating information, getting involved with and understanding the service or product, and understanding the financials of the product or service. By financials I am not referring to having a CPA before you undertake the task, but rather understanding the presentation of the information and analysis/ numbers to support the activity being planned. Financials are important because they are the score card in the world of commerce.
Would you ever think of taking a trip to a foreign location without some type of tourist guide or map? Could you imagine doing this and then the feeling of suddenly realizing that you have no idea where you are or what to do? I would imagine that would be a terrifying experience. Fortunately people take trips like this all of the time and these types of experiences are averted through proper planning. Operating a business without a business plan is like taking a trip without a map. You do not want to dedicate precious time to building a business only to suddenly realize that you have no idea where you are or what to do.