I've gotten a lot of questions about Lutheranism over the years. Since I'm not a Lutheran, I have to admit I only have a cursory knowledge of their theology and its historical development. Why do Lutheran believe salvation can be lost? Why do the Reformed say it can't?
I recently came across a few paragraphs from Reformed theologian Geerhardus Vos about this. Ponder this, as I have been doing:
When we compare the representations of the on final state of man as they have been developed by the different theological traditions, there immediately arises a fundamental difference of great importance for the doctrine of the covenant of works. According to the Lutherans man has ready reached his destination in that God had placed him in a state of uprightness. Eternal life was already in his possession. In his situation the highest ideal was realized. Nothing more need be added to execute God's purpose in creating man. Man was mutable, that is true, and he could fall away from the state of original uprightness and bliss. But for the Lutheran conception this is not a stage that points forward to something else, but rather that which was usual and normal and to be expected. From this it follows that the same condition returns in the state of grace to which fallen man is brought by Christ. Precisely because mankind's destination had already been reached before the fall in Adam, Christ can do nothing but restore what was lost in Adam. And since the destination already realized was fully compatible with mutability and the possibility of falling, the sinner who has been brought back to his destination by Christ must necessarily have to remain at this level. Lutheran theology is, therefore, wholly consistent when it teaches an apostasy of the saints. It does not at all object to uniting the state of justification and sonship with the possibility of such an apostasy.
-snip- (I 'm skipping the discussion of the Pelagian view) -snip-
The Reformed view of the original state of man leads to a totally different result. It was a state of perfect uprightness in which he knew the good and did it consciously. As long as he remained in that state, he could also be sure of God's favor. Up to this point the Reformed view concurs with the Lutheran. But whereas the latter can be satisfied by perpetuating such a state and extending it indefinitely, the Reformed view fixes its gaze on something higher. It sees man not as being placed in eternal bliss from the beginning, but as being placed in such a way that he might attain to eternal bliss. There still hovers above him the possibility of sin and death which is given with his mutable freedom. He is free to do the good out of his good nature but he has not yet attained the highest freedom which can do good only. The latter is placed before him as an ideal. The means of obtaining it is the covenant of works. Here too the state of grace is again ultimately determined by the idea of man's destiny in the state of original uprightness. What we inherit in the second Adam is not restricted to what we lost in the first Adam: it is much rather the full realization of what the first Adam would have achieved for us had he remained unfallen and been confirmed in his state. Someone placed in that state can never again fall from it. As truly as Christ is a perfect Saviour, so truly must he bestow on us the perseverance of the saints.
Source: Richard Gaffin, ed. Redemptive History and Biblical Interpretation, The Shorter Writings of Geerhardus Vos (Philipsburg: Presbyterian and Reformed Publishing co., 1980) p.242-243.