I have just been discussing music with a friend. More specifically the covering, or playing, of a piece of music you did not compose yourself. It hit me how similar music and aikido really are.
If someone makes a cover, they will try to give the same feeling as they pick up from the original piece. At the same time they will give their own tweak on it (intentionally or unintentionally). However, exactly which feelings they receive most strongly from the original can be individual. Thus, we might end up having very different opinions about certain covers, depending on which parts of the original we pick up more strongly (all of which exists in the original of course).
This is probably how we ended up with different lines of aikido. The closest students of O sensei received certain parts more strongly than other parts and since they are themselves different persons the main feelings they picked up varied slightly, and therefore they started developing in different directions. So we (the aikidokas of our time) are now free to choose which line (or style if you wish) to pursue.
The same ting happens when a bunch of friends go together to seminars. Just as with the music, we will receive different parts more strongly than others. That is why it is so great to travel together to seminars and do keiko together afterwards exchanging the parts we were able to pick up.
Enjoy your keiko! Aikido make people happy!