Variances are always positive because they are the average of squared numbers.
The magnitude of the variance determines the average squared distance from the mean of the distribution. We denote the variance of the random variable x as sxx. The square is written as a product to conform to a general format that will be useful when making comparisons with the covariance. It is for this reason that sxx is sometimes written as sx2. Once again, the sxx format is preferred because of the ease of making comparisons with the covariance.