Think of mining like rolling a dice with billions of faces.  The chances of the dice rolling in your favor is very low, even if you have a very fast rig.  If you join a pool, instead of your 1Ghash/s+ you will have many others helping you roll the dice also.

If your rig can find an average of 1 block every 2 weeks, you will need at least 30 to 60 weeks for your luck to even out.  Any less than that and you risk the chance of being normally lucky, very badlucky, or very lucky.

In either case, the Expected Value of each option will be the same, but by joining a pool you can normalize your luck within a few days to ensure a constant predictable flow of money.