Percy had always been an outcast in his family. A stick in the mud. Bill and Charlie had eachother and by the time Percy came along there was too much of a age different between Charlie and Percy. Fred and George had each other and spent all their time pranking everyone. Ron was too much younger to get along well and with Ginny coming along a year later Ron hung out with her. Leaving Percy without anyone in his family to hang out with. He retreated inside himself and without someone to hang out with he got memorised with his work and his job.