Egg Groups
Egg groups mean our Pokémon lives are much easier than they otherwise would be, after all who would want to search out two of a really rare species just to breed them? If you really think about it, though, they make no sense at all. After all, rats and snakes can't breed in real life, so why can Rattata and Ekans? Whatever disturbing images this might produce, you can't deny that they can breed. But why is this? What caused this complete disregard for mating laws?
The reason for this is that they all evolved (in the realistic sense, rather than the Pokémon sense) from Ditto. This evolution formed one Pokémon for each egg group, which then further evolved to form all the different Pokémon species. This would explain why all Pokémon can breed with Ditto, as well as why there are often some similarities between Pokémon in the same egg group - other than the obvious of that being the reason their categorized in a particular group. Much as a tiger can breed with a lion, these Pokémon can breed with each other.
What, then, is the explanation for those Pokémon that belong to more than one egg group? In the earlier stages of the original Pokémon's evolution from Ditto, some breeding across egg groups occurred (they were still similar enough to Ditto for this to happen) which led to the dual egg group Pokémon of today.
And finally, why do Pokémon always inherit their mother's species rather than their father's species? Rather than having two of a gene, one from the mother and one from the father, they have one of each gene - one that decides their species, one that decides attacks, etc. And these genes are designed such that the species gene is always inherited from the mother, the attacks gene is inherited (to an extent) from the father (hence egg moves depending on the father's moves), etc. Thusly, it is guaranteed that a Pokémon will always be its mother's species.
Egg groups are an entirely nonsensical idea, but they can be easily explained with a little bit of amateur science. They'll never make complete sense, but hopefully their nonsense is a little more sense-making now. The end~
