in humanistic psychology, there is a concept that actions can be self-determined or controlled.
-self determined behaviors are those that have an intrinsic value to us
-controlled actions are those we do because of a reward or to satisfy pressure

how long we stay interested in a certain behavior depends on whether we view it as controlled or self determined - it's been shown that people stay more interested in actions they feel are self determined

in a study done in the 1970's it was demonstrated that when children were excessively rewarded for drawing (something they enjoy doing naturally), they lost interest in it. this is called "turning play into work".
when the children felt that drawing became a controlled task, a job, it wasnt interesting anymore.

it seems to me that if we feel that our torah observance is self determined we will stay much more interested in it than if we feel it is controlled.. even more important, i think that children who are taught to view their torah observance as a controlled behavior might eventually lose interest in it..

believing in a system of reward and punishment is part of a torah perspective - it isnt up for debate...
but... the motivation behind our torah observance is for us to decide. do we follow the torah so we can get a reward and avoid punishment or do we do it because we feel that there is an intrinsic value in it?

if we want to feel a sense of self determination in our lives, that we are choosing how to live our lives and that our choices are autonomous and not imposed, we need to start viewing our behaviors that way.

regardless of whether someone was raised religious or not, at some point in our lives we all need to "choose" our behavior, and more importantly, we need to choose it for the right reason - not because we feel pressured or guilty and not so we can get a reward but because we view our choices as being intrinsically worthy.

teaching kids that every mitzvah/aveirah they do will get a "reward" or a "punishment" is reinforcing this idea that torah observance is a controlled behavior...
in addition, humans have a need to receive unconditional affection. if we feel that affection - from parents, teachers, ourselves and most importantly from G-d - is dependent on conditional behavior, we risk losing interest when it gets too difficult...

so.. of course positive behavior should be rewarded and encouraged, but not excessively and not as the only motivator. in order to feel self actualized and a sense of autonomy, we have to remind ourselves and emhasize to children that there are more important and more intrinsically valuable reasons for keeping mitzvot
until we internalize this...we risk losing interest..

(one thing i want to mention is that im not refering to anyone who feels that they are observing thetorah because of societal pressures - thats a whole other (bigger issue). what im referring to here are those who want to be torah observant but dont always enjoy it or appreciate it or feel self actualized in it)