I have heard it explained in the following way, particularly in regard to the Eucharist:
The Catholic Church believes that ordination grants the bishop the power to change the bread and wine into the body and blood of Christ. This power still exists, even if the bishop breaks communion with the Catholic Church. By the laying on of hands, the bishop can pass this power on to others, and “valid” sacraments can continue, as long as the succession is maintained. This is why, from a Catholic point of view, the Orthodox can still have a genuine Eucharist.
By contrast, the Orthodox Church believes that the sacraments belong to the Church and work within the Church. If a bishop leaves the Church, he does not take his sacramental power with him, and he is essentially a schismatic layman. This is why, from an Orthodox point of view, the Catholics may or may not have a genuine Eucharist. There is not an absolute statement on this, but this is why the Orthodox cannot state that the Catholic Church has genuine sacraments. They are not trying to be mean. They cannot reciprocate the Catholic Church’s affirming view of their sacraments, because they have a different idea of how this sacramental power is maintained.
Again, that is just my understanding, and it may be incorrect.