This is a large part it. Lucas doesn't appear to have thought it too well over, but that's what he is presenting in the movies, especially III, V, and VI. Attachment can lead to "negative" emotions, such as fear or anger, which can lead to the Dark Side.
Certainly, the policy of forbidding attachment entirely doesn't seem rationally or ideally sound, but that's the idea posited by Lucas for the Jedi. It may seem like a bit of overkill, but despite how the threat may seem manageable to us, Lucas wants it to be very real and dangerous for them.
Probably the hardest part of understanding fiction when you start really looking into it is realizing that the same rules we see from the real world don't necessarilly apply. These characters don't really have a psychology. Now, I'm not saying it's normal, in Star Wars, for people to not feel love. But, that's it's more manageable for Jedi, presumably due to stronger discipline. It may seem overly controlling to you, and in the real world you'd likely be right, but within context of what the universe has shown, the rules are there for a very good reason. Fear of losing people you love is the very thing that Palpatine exploits to turn others to the Dark Side. And it ruined the galaxy.