Waddell’s book Formng Intentional Disciples does not really answer the question about what exactly a personal relationship with God is.
For some people, personal has an erotic connotation – up close and personal. The unfortunate tradition of erotic mysticism uses this sort of language to describe the Christian’s relationship with God - Christ as the Bridegroom of the soul, falling in love with God (language Waddell uses) and even more explicit language and images (think of Bernini’s St. Theresa). Perhaps some people (especially men) do not like this language and that is why they do not use us the word personal to describe a relationship with God.
Also, our relationship with Jesus is not like that with another human person. Unlike those who met him while he was on earth, we, like Paul, do not know Jesus according to the flesh but according to the Spirit. Jesus is glorified now, and pours forth his Spirit through many channels, both ordinary (Scripture, the sacraments) and extraordinary. This is not the sort of personal relationship we have with other human beings, so again, that may be a reason Catholics do not think it is possible to have a personal relationship with God in Christ.
If a person believes in God, and says his relationship with God is not personal, what kind of relationship is it? I find it hard to imagine it’s being an impersonal relationship, the type we have with the force of gravity, or the US government.