Over what period of time are you referring to - a day, a week?I once read that of a stock's move, 31% can be attributed to the general stock market, 12% to the industry influence, 37% to the influence of other groupings and the remaining 20% is peculiar to the individual stock.
If you are a trader or you are purchasing a tracker - sure the level of an index is important. From your ID, I'm guessing you are interested in trading, possibly using charts (technical analysis)???
As an investor, I'm only interested in the company I'm buying. Does it fulfill my investment criteria? is it at a price that's attractive to me? The level of some market index is completely irrelevant. I'm buying the future earnings of a company, not a ticker symbol or an index. The last company I bought shares in (December) was a rapidly growing German financial services company. PEG ratio 0.5, excellent management, very profitable, no debt, growing market and at a wonderful price. I spent about two months analysing the company before deciding to buy. Most likely I will still hold the shares in five years time. Did I look at the German DAX index? No!
Back to phoenix_n's question - If an inexperienced investor wants to sleep at night, borrowing to buy shares is not a good idea. Borrowing to trade is crazy unless you're very experienced.