There are two reasons: First, I began and nurtured a love for reading; second, there was nothing else worthwhile to do.
I didn't develop my love for reading in high school. It wasn't in my school's culture (this somewhat explains why English was a troublesome subject for most people then).
I did not go to novels immediately. I began with comic books much earlier, and I didn't consider that real reading. I read my first book before I started 4th grade. It was a Bobsey Twins book. I thought that was a breakthrough. It took me a long time for me to finish it.
Then, there was the matter of not having anything much to do inside the house during summer in the mid to late 80's. Eventually, I finished all Bobsey twins books. And then all Hardy Boys. And Nancy Drew. And Dana Girls. They were all from one author (well sort of: the author was the Stratemeyer syndicate under four pseudonyms) so the plot formula and writing styles of all the series were the same. My aunt was also the school librarian and I even borrowed the much older books in those series from the school library.
Eventually though, my reading interests branched out and I finished almost every reading material in the house -- save for the reference books. I remember Frederick Forsyth's Day of the Jackal for instance. But I didn't immediately jump into more "mature" novels. They didn't seem all that interesting and reader-friendly to the average 10-year old.
I went through everything. I even picked up every old periodical I could find: Reader's Digest, Newsweek, Time, etc. that my dad either subscribed to or simply brought home from the bank. Some of them were really old. One afternoon, I walked into the kitchen while my sister was cooking a snack. Then I asked her: Who's the real Soviet premier -- Khrushchev or Brezhnev. My ate looked at me with a curious grin and said, "Gorbachev." Yeah, some of those Newsweeks were really old.
There were summers also when my sister and I would go out and rent pocketbooks at the local boutique stores called Puno Arcade. That was were my dad bought me all the Peanuts books back in fourth grade (I read all the Snoopy ones first). I consumed several novels each week for a couple of summers in high school. Sure there were other activities, numerous school clubs and sodalities, Taekwondo, Citizen Army Officer's training, but I kept on getting my hands on books whenever I could.
It's not that my social life was that bad. I had some good friends and I hung around with peers like most kids. But there were summer days when one of my friends, Paolo Arugay, would be in my house and he'd ask me: Didn't [insert name of cute girl] live in this village? I'd say, sure, I could point you to her house. He'd tell me to get up so we could swing by. I'd tell him to go ahead alone. I was reading.
I read most of my English and Reading books before school even started. Sometimes more than once. On some boring weekends, I opened volumes of the encyclopedia at random. I constantly revisited the articles about dogs, cats and snakes (those had the coolest pictures). I read up on the Theory of Relativity (not that I actually understood most of it) while I was still in sixth-grade science.
Around freshman year in high school, I started reading volumes of a medical encyclopedia we had at home. I don't even know why we had those; we had no doctors in the house. Around the topic "aneurysm" in Vol. 1, I remember skipping to anything related to sex and the reproductive system (I had never seen a porn film in high school and the stuff some friends were feeding me sounded unbelievable).
I loved fairy tales. I finished volumes of them -- the original gritty ones, not the wimpy-ass Disneyfied ones -- several times and some I still know by heart. I went through the four Gospels several times also -- though I only started reading the Bible so I could get drowsy enough to fall asleep.
One of my favorite books was called Natural Wonders of the World -- a Reader's Digest compilation (as with the fairy tale books; my father was into Reader's Digest compilations). Because of that book, I can identify various sites around the world like Angel Falls, Crater Lake and Wizard Island, various rock formations in Arizona, etc. I taught myself a little geology -- sinkholes, how desert and beach sand particles differ, cave formations -- all of which was exhilarating for a 12-year old thirsty for knowledge.
I read short stories by Hemingway, Chekov, etc. Really good stuff. I read Dekada 70 by Lualhati Bautista. I never saw the movie version but I really recommend the book.
I read typical adolescent books, of course: Choose Your Own Adventures, Time Machine, etc.
There was a time around high school sophomore year when I really wanted to play TV console games instead of reading. I wanted a Nintendo Family Computer -- if anyone can remember those. My mom was about to buy me one, till another parent discouraged her from doing so because of school work. I was bargaining and arguing with my mom that the Nintendo would not prevent me from studying on school nights because I did not study at all on school nights -- except for major exams. Impeccable logic, I thought, but my mother didn't buy into it.
Years later, I was thankful that my mom didn't buy me the Nintendo. I learned during college that I was actually a compulsive computer gamer -- and a pretty good one, I might add. I stopped gaming years ago, but back in the day, I could beat everyone in Mortal Kombat (1, 2 and 3). I went through Doom 2 without God Mode or any form of cheating (well, ok, I pressed IDKFA to load up on weapons sometimes). In the last years before I stopped turning on my Xbox, I pummeled everyone in Halo 2.
If I started gaming in high school, it's possible that I wouldn't have nurtured my love for books. I really believe that.
And you must love books. The tactile experience of opening a book and hearing the rustle of pages as you turn each leaf cannot be replaced by any digital medium we now have and live with. It's not that I am against digital media. The web is one of my passions as well as my bread and butter. But I would urge everyone to appreciate the older forms. Trees died even for those books that we take for granted.
With most things in life, it's worth noting that I got into the things because (a) I had a natural love for it and (b) the environment was conducive to nurture those interests.
As I alluded to earlier, I did not get into girls and dating until much, much later. This explains how I got stuff done.