People always confuse galling with galvanic corrosion, which is when two (untreated) dissimilar metals mated together react. The farther apart they are in nobility (anode vs. cathode), the faster they will corrode (electric discharge from positive to negative material). Adding a electrolyte (like water) speeds up the process. Check out the chart below. You'll notice that Ti (and slightly less so steel) is at the far end of the nobility scale from aluminum. That's why they corrode and stick together so quickly. You should always Use some form of prep between the two metals. It forms a barried between the materials and slows the process (plus, it helps keep out the electrolytes like water by forming a seal).
PSA: Most commercial Ti prep compounds contain crushed glass. You should avoid getting it on yourself, and wash it off immediately if you do. The glass particles will go right into your skin, taking any chemicals mixed in with the prep compound along with it.