I think the real question is how you get from a meaning of strike to infatuated. When there is semantic shift that causes a new sense of a word to splinter off, there is usually for some time some figurative meaning of the old sense that eventually starts to get popular and takes on a life of its own.
So if you look at the original sense, you'll see that it is also paraphrased as afflicted. Semantically speaking, smiting (a physical blow) has agent-patient roles, while smiting (afflicting) has stimulus-experiencer roles. Those are the seeds of the semantic shift.
Now affliction can be physical or it can also be emotional. Look at these uses of the word from the 16th century, and you see it's a smooth cline from a physical stimulus (leprosy), to a stimulus of danger (fear), to a romantic stimulus (temptation), to a general longing/soul searching (self-doubt).
1545: mary through pride and inobedyence murmured agaynst her brother moses, and annie: xii: by and by she was smitten with the lepre: this mary that rebelled agaynste moses that is againste her prelate,
1543: the bishop couth full mekill skyll, of a woman alwaye that so couth chese, a lady that was vncouth and for yt mery woordes, that came of his mouth Thei trowed he had, right great experience of womanes rule, and hir conuenience Kyng robert bruys, smitten in lepry dyed to whom his soonne dauid, then did succede and crouned was, for kyng and notified his wife also, was crouned quene in deede Kyng edwardes suster
1550: and my fleshe is smitten wyth feare
1558: dauid dyd behold curiously the beauty of his souldiours wyfe, and sodeinly he was smitten in the hart with the dart of adulterye
1559: another of his exercises was this: he vsed to make vnto himselfe an ephemeris, or a iournall, in which he vsed to write all such notable things, as either he did see or heare, ech day that passed: but whatsoeuer he did heare or see, he did so pen it, that a man might see in that booke, the signes of his smitten heart: for if he did heare or see any good in any man, by that sight he foud and noted the want thereof in himself, and added a short praier, crauing mercy &; grace to amend: if he did heare or see any plague, or misery, he noted it as a thing procured by his own sins, &; stil added, domine, miserere mei, lord, haue mercy vpo mee:
It's plain to see how the meaning of the term gradually morphed, and I think you can see obviously metaphorical uses of the term in the 16th century, well before OED is ready to concede a second sense.
But to actually get to the second sense, you need different argument realization as well. Look closely at the last two examples. Both have the smitten person's heart playing some role in the sentence. I'd guess that the phrase smitten in the heart or something like that eventually became conventional to the point where you could omit in the heart. Then you are clearly at a new sense, because the argument frame has changed.