Can people from healthy families develop attachment disorders?

Is is possible for children raised in more-or-less healthy, non-abusive families to develop attachment disorders?

My parents weren't perfect, but they didn't make any huge mistakes that I know of. My mom worked part-time when I was a toddler and then became a stay-at-home mom after my younger brother was born. My mom breast-fed me as a baby, and my parents were both fairly affectionate. I don't remember any abuse or neglect, and my parents are still married.

Despite that, as a young adult (24), I find that I can't get emotionally attached to anyone. I was very quiet and unsociable as a child and teenager. I learned some social skills as a college student, but I still can't get close to anyone. I try to make friends, but they always start to annoy me when we start to get close. I went to a psychologist a couple of years ago and was diagnosed with Generalized Anxiety Disorder and "avoidant personality traits".

The thing is, these problems started when I was very young. My parents say I was very agreeable as a child, but I was also very quiet and "in my own world". As a young child (about 4-6 yrs. old), I remember being happy but never interested in talking to people. I daydreamed constantly, and also started having sado-masochistic fantasies around that time, although they were not overtly sexual at that point because I didn't know about sex.

The rift between me and my family started around age 7. That is the first time I remember thinking to myself that there was something wrong with me (because of my addiction to daydreaming and fantasies), and that I couldn't tell my parents because they would think I was crazy. As a preteen, I went through several stages of anxiety which I never talked to my parents about, and by the time I was a teenager, I had low self-esteem. I started developing mild depression symptoms after I went to college, and they have gotten worse ever since.

I very never dated (other than one short and pointless relationship when I was 18) or had sex. I have problems with sexual addiction because I can't stop indulging in sado-masochistic fantasies and masturbating, but I have never met anyone I was attracted to or wanted to have a normal romantic-sexual relationship with. I have been reading a self-help book on sexual addiction, and it says these problems are often caused by problems with intimacy, which is often cause by abuse, trauma, or neglect.

I can see how the rift between me and my family makes it difficult for me to be intimate, but I think my problems started before that. Does anyone know if it is possible for attachment problems or fear of intimacy to appear in children without some sort of trauma?

Or can these problems have natural, biological cause? I have researched autism a little bit, and while I identify with autistic people in some ways, I am pretty sure I am not autistic. If the problem is caused by inborn traits rather than trauma, what can I do to overcome this?

Thanks for any advice or experience anyone can share.