Absolutely yes to the socializing and introverted angle. My DD found crowds a lot harder than one on one and so do I. She still prefers, at thirteen, small groups or one on one. It has a lot to do with being reserved and the additional energy required to break into an established group. Even at daycare she would hang on the periphery and survey the scene until she knew what was going on, every morning despite daily attendance for two years!
As a reassurance she has friends and is not actually shy...she just does what she feels comfortable doing. As she is getting older she is more able to manage group situations. Drama classes, gymnastics, martial arts have all been engaging activities for her where she can either do her own thing alongside others, the latter two, or move outside her comfort zone in a happy way (drama).
As to the obsession thing.....we have had serial interests rather than one prolonged one. I actually think the interest is good as it is driving him to develop skills so he can better learn about the cars which helps him have a reason to learn, be it colours or numbers or letters. It's great to know if you leave books lying around to do with cars he will progress his reading, he can learn science in the context of cars. Maybe he will grow up to design or develop cars, or maybe he won't but it's fine, apart from driving you a bit nuts. That's always a bit of a drag!
I saw a great science kit a while ago showing kids about car motors....see if you can find something like that.
