We study the volatility time series of 1137 most traded stocks in the US stock markets for the two-year period 2001-02 and analyze their return intervals τ , which are time intervals between volatilities above a given threshold q. We explore the probability density function of τ , P q (τ ), assuming a stretched exponential function, P q (τ ) ∼ e −τ γ . We find that the exponent γ depends on the threshold in the range between q = 1 and 6 standard deviations of the volatility. This finding supports the multiscaling nature of the return interval distribution. To better understand the multiscaling origin, we study how γ depends on four essential factors, capitalization, risk, number of trades and return. We show that γ depends on the capitalization, risk and return but almost does not depend on the number of trades. This suggests that γ relates to the portfolio selection but not on the market activity. To further characterize the multiscaling of individual stocks, we fit the moments of τ , µ m ≡ (τ / τ ) m 1/m , in the range of 10 < τ ≤ 100 by a power-law, µ m ∼ τ δ . The exponent δ is found also to depend on the capitalization, risk and return but not on the number of trades, and its tendency is opposite to that of γ. Moreover, we show that δ decreases with γ approximately by a linear relation. The return intervals demonstrate the temporal structure of volatilities and our findings suggest that their multiscaling features may be helpful for portfolio optimization.