Article you quoted is rather simplistic and sensationalistic. Napoleon invaded Russia to force her to join his new European order (like other European nations, Austria, Prussia etc ..), especially his Continental System which was essentially economic blockade against Britain. Russia at first joined this system, although halfheartedly, but Napoleon wanted to strengthen it. Russians still treated with Britain trough third-party intermediates, and Napoleon wanted to stop that. Also, Russians refused proposed Napoleon's marriage with Grand Duchess Catherine Pavlovna (he latter married Marie Louise, Duchess of Parma). Napoleon wanted to legitimize his status of Emperor by marrying into established European royal families, so this move by Russia offended him. Finally, from strategic perceptive, Napoleon wanted to strengthen Duchy of Warsaw (Poland) created by him, by incorporating some territories held then by Russia.
In order to achieve his political aims and force Russian Emperor Alexander I to accept his terms, Napoleon planned to either decisively defeat Russian army (i.e. "destroy it") or capture one or both Russian capitals (old Moscow, or new St Petersburg) . Napoleon crossed Russian border on June 24th, 1812 and moved east. Initially, Russian army planned to meet him at their fortified Drissa Camp, but they abandoned that plan because of their numerical inferiority. Also, at the beginning of the war, the Russian First Army (Barclay de Tolly) and Second (Bagration) were separated, which meant that Napoleon could defeat them in detail (these goes especially for weaker Second Army). In fact, first part of the campaign was actually game of cat and mouse - First Army retreated eastwards, Second Army attempted numerous times to join them, and Napoleon and his generals attempted several times to destroy Second Army. These attempts were quite costly for both sides, but most of these battles are almost forgotten these days. One example would be the Battle of Saltanovka, failed Bagration's attempt to link up with de Tolly, that left thousands of dead in the battlefield. Anyway, as Russians moved towards Smolensk and Moscow, Napoleon had to follow them. This in fact determined direction of campaign. On northern route, Russian army under Wittgenstein and French under Saint-Cyr essentially locked themselves into stalemate until October, thus St Petersburg was not threatened, and this become secondary theater of war.
Now let's focus on those first months of the campaign. Napoleon's army by various estimates had around or more than a half-million men (not all of them marched with Napoleon though). It was enormous logistical effort to move them across relatively poor roads of the Russian Empire. Cases of illness and death were to be expected in poor sanitary conditions and with undeveloped medical science. Nevertheless, those same conditions affected Russians troops also. Here is detailed account of events in June and July. By July 28th, mentioned in the article, Napoleon was in Vitebsk. There is no mention how many troops he had lost so far, but that didn't bother him because he was still much stronger then Russians. In fact, his concerns at the time were how long would campaign last :
The first Russian campaign is over… We shall be in Moscow in 1813, [and] in St Petersburg in 1814. The war with Russia is a three-year war.
He certainly had an option of stopping in Vitebsk, but that would not fulfill his strategic goals. The Russian army was not decidedly defeated, although lot of territory was captured, none of the Russian major cities was occupied. And yet, few more days of march to the east was Smolensk, with opportunity to finally get the decisive battle he sought. At that point of time the Grande Armée was certainly capable of moving forward, to stop it now and accept losses taken so far would be completely un-Napoleonic thing to do.
Finally, as both armies were getting smaller (due to battlefield attrition, diseases, desertions etc ...) it was easier for Napoleon to control his forces. He now had to focus only on one opponent (always numerically and tactically inferior to him until after Maloyaroslavets), instead of worrying about divided forces that had to fight multiple enemy formations.
