Step 1: Determine your goals
Step 2: Determine your risk tolerance
Step 3: Create a portfolio, within your risk tolerance, that is designed to achieve your goals.
Step 4: Help you stick with your portfolio through periodic re-balancing and telling you to hang on during market declines.
This sounds fine in theory, you have a goal or goals that you want to achieve and if you can design a portfolio that will give you the best chance of achieving them then what is wrong with that?
What is wrong of course is that the market doesn't care about your risk tolerance or your goals, it will do what it is going to do. For example, lets say it is the year 2000 and you are 55 and want to retire in 10 years. Some simple math tells you that you need to average 10%/yr in your portfolio to reach your retirement goal. Anyone can design a portfolio for you that would have returned 10% over the past 10 years but that is meaningless for what it is going to do over the next 10 years. In fact, you probably would have reached 2010 with about the same amount of money you started with, if you were lucky. So in effect you are getting an asset allocation that a financial planner is guessing might have a chance to earn 10%, you are re-balancing to that allocation, and being advised to stick with it during market downturns.
Agility Is What Matters
Would you ever write a business plan and stick with it no matter what? No. New opportunities and new threats always pop up, your business plan needs to be constantly updated to keep up with reality. Why would you ever design an investment plan and stick with it? In the above example, since I have no way of predicting what mix of assets will return 10%/year the best solution is to earn as much as possible while taking a risk level that you are comfortable with. This requires agility and the willingness to respond to new opportunities like Bull Markets, and new threats, like Bear Markets.