Riding Solo all the time

I can't give you a scientific response, but I can say, from the perspective of someone who primarily trains alone in all three sports, that when I do workout with a buddy, I seem to be able to go faster. I noticed this well before picking up triathlon when I was just a runner. I don't know if 95% of my workouts are alone, but the vast majority of them are. When I get out on the road to run or ride with a friend, I just go faster. I think part of it has to do with having a friend to distract you from the pain. Naturally, part of the benefit comes from any drafting that may take place. I'm not sure what the rest is from though.

 So no, you're not alone in noticing big differences when you're out with someone else.

There are several ways that riding with the other person can make things easier: 

1. There can definitely be a "social facilitation" effect, which is the added boost you get just from having someone else around.  Maybe it's competition or maybe it's instinctive cooperativeness. 

2. Having someone to talk to is a nice distraction. Distraction can be helpful at moderate level of effort (at intense effort, a more focused concentration may be better). 

3. Then there is the whole drafting effect, which can save an enormous among of energy.  The best position isn't always behind the other rider. In the cases of cross winds, the best position is to the leeward side of the other rider.

I train mostly alone and search out for other riders or runners to run down.  I'll even go out of my way to catch up and blow by someone, just cause.  I can only think of 2 times that someone did it to me.  Once I was really struggling and a grandma could pound me.  Another was when I was riding very solidy and got beat by someone really better.

I love the rabbit aspect of riding and running.  It is motivating to catch and pass someone.  It can be distracting to be passed however.  I try to counter it by thinking, 'youve been following me and I did not know you where there. Now's my turn to chase and catch you'.

sorry...

...thought you meant Hope Solo...

You're absolutely not alone on this one.  Every ride I've ever been on with someone else (which has only been 3 or 4) I've been able to go faster with less perceived effort, same for the run and swim.  Don't know what it is exactly, maybe it's just more fun to share this with other people.

I've also used the "chasing the rabbit" technique on long solo rides.  It helps keep me motivated when I start to get tired.  I must say that I prefer riding with someone rather than chasing them down, but that's pretty fun too!