If you're married, you will understand this. If you're engaged, you and your fiance are probably experiencing it right now. If you aren't looking at getting married in the immediate future, prepare yourself. What's one of the absolute worst parts about planning a wedding?
People inviting themselves. People that are not on the guest list.
When a bride and groom make their guest list, they are usually limited by the size of their venue or their budget. I know from experience, it is SO INCREDIBLY DIFFICULT to not include everyone. Imagine how uncomfortable a bride feels when she is approached with "Am I invited to your wedding?" or "I cannot wait to go to your wedding!," and the person isn't on her list.
Even if you are 80% sure you will receive an invitation, don't put the bride on the spot. You don't know the circumstances of her guest list, budget, venue restrictions, etc.
If you have never put together a guest list before, I cannot begin to describe the heartache involved. When Aunt June wants her best friend to come along, but you'd rather invite your best friend from high school... A girlfriend you haven't talked to in 4 years wants to come, and you barely keep up with her through social media... Your cousin wants to bring his girlfriend of 2 weeks, even though you didn't include a plus one on his invite... A last minute 'yes' is given, even after the RSVP date... I could go on for days. Would you want someone to say yes to you out of guilt? Yeah, me either.
I recommend examining your motives when you ask the bride if you will be included in her special day. Of course, every bride and groom I know would invite everyone they knew if they had the endless funds to do so. In reality, there will always be limitations, so go easy on them. Engagement is a naturally stressful time. Instead of perpetuating the stress, it's best to see any potential invitation as a privilege, not a right.