I'm perplexed - why do all these outside creatures think they can take refuge in my house?

Last night it happened again.  Ugh!  I noticed Oscar (my cat) was under the kitchen table intently staring at something and occasionally swatting at it, so I went over to see what it was.  It turned out to be a slug.  And this was not the first time.  In fact, over the past 6 or 7 months, I have had so many strange bugs and insects, that you usually do not find inside your home, that it has me scratching my head.

I've lived in my house for nearly 16 years, and knock on wood, I've never had any of the usually disgusting bugs in my home like cockroaches, waterbugs, or the like.  But, if I did, I wouldn't like it, but could understand because that's not totally unusual to see them in one's home.

However, I've now had 3 slugs.  Here's the evidence of the first one I found.  Apparently, Oscar found it first.

I've had several moths in my house for the first time since I've lived here.  Some of them rather large.  Here's one that Oscar stunned and got it on it's back.  I don't know if you can tell from the pic, but that moth was about 3 inches long from it's head down.  As a reference, Oscar's whiskers just on one side are about 6 inches long. 

I've had a number of bees and wasps.  These are most upsetting because they also seem the most difficult to get rid of - not wanting to get stung and all.  Here's a rather large bee I found one morning (I think it's a bumble bee?). 

Then there's the unidentified flying insects.  Here's one that was hanging out on the window - I have no idea what it was, but it was pretty big and I didn't like the looks of it.

And lets not forget about the time that I went down to my garage this summer and found a huge bird's nest in there.  I have no idea how they got in there.  There are no holes or gaps that I am aware of for them to get in there.  And this nest was pretty big - probably about 18" across.  Wouldn't that be a larger size bird?

All of this is just so confusing to me.  Just seeing that slug last night really ticked me off because I have no idea how these bugs are getting in my house.  Especially since last year, I had the stone around the bottom of my house repointed and the foundation where it meets the ground was all resealed.  I also had all the windows recaulked.  Inquiring minds would like to know?  If anyone has any thoughts I would love to hear them - I am open to any and all suggestions.  I do not want to go through this again next summer.  I'm not a super girly girl, but I really, really hate bugs!