10 is a lot and by the time we get to the end we'll be grasping at straws, but to take some of the main points in order, and bearing in mind that most of these aren't so much "reasons Sea Lion must fail" as "reasons why Sea Lion is unusually challenging as a military operation," which, taken all together, raise the difficulty level well beyond any sense of feasibility:
1.) German planning is premised on the comprehensive defeat of the Royal Air Force BEFORE the invasion can begin. Obviously, that didn't happen. Without air superiority, the Germans themselves think it's a no-go.
2.) The Germans do not have a proper, purpose-built amphibious fleet. This means they have to strip their inland waterways of small cargo craft and hastily refit them to carry troops, horses, and vehicles. This makes the invasion fleet far slower and less seaworthy than anything the Allies ever mount an operation with.
3.) It's unlikely the Germans can get this assemblage to the invasion beaches without being detected. Obviously, regardless of the level of preparation, lack of surprise is always an extra challenge.
4.) The Germans do not have good naval cover for this fleet. A large proportion of what should have been the escorting destroyers were already sunk. At least in terms of destroyer class and up, the Royal Navy will outnumber the Germans quite literally by an order of magnitude.
5.) Even if the first wave gets through that fortunately unscathed, the likelihood of each next resupply and reinforcement waves making it through, given points 2 and 4, rapidly diminishes towards zero. In short there is a very high likelihood that any army units making it ashore will be cut off.
6.) It's unlikely that the Germans can resupply such a beachhead by air, but to the extent they try, it will necessarily draw away resources they would have been using to advance on the ground, fight the RAF, and fight the RN.
7.) In theory the Germans can build up their forces to address these problems after Dunkirk, but they can't do so in a vacuum. The longer they dither, the better prepared the British will be. The temporary disarray after Dunkirk will be repaired by the fall, and it's obviously impossible to do this in winter, and then you're into 1941 already.
8.) In theory the Germans can also build up their forces to address these problems BEFORE Dunkirk, but again, they can't do so in a vacuum. Not only will the British be likely to step up their game in response, but it will draw away resources needed for the invasion of France, and if a sudden obsession with defeating Britain scuppers the invasion of France, all of this is a moot question anyway.
9.) Always going on in the back of the German military mind is that Russia is the easier target anyway. With hindsight this seems absurd, but remember, Germany had already successfully invaded Russia once, in the First World War. Successful invasions of Britain are in contrast not common. This is less of a reason why the invasion would fail then a reason why Germany wouldn't launch it to begin with.
In naval clashes of the period - Nazi warships were twice as effective as Wallies.
Cold comfort when you're outnumbered 10 to 1.