An English mastiff is going to be a lot more than 100lb - more like 80 to 100 kilos.
Mastiffs are delightful dogs and usually have wonderful temperaments. They are calm, quiet (most of the time) sensible and dignified dogs.
I have a friend with an Bull Mastiff and a friend who breeds GSDs who used to breed Mastiffs.
We have 3 GSP x Mastiffs 2 of which have the build and temperament of GSP (except one of those is very tall) and one that has a heavier, far more solid build and a bigger, wider head. He has also inherited his mastiff father's temperament.
If you start training a mastiff puppy from a very young age you will have loyal sensible well behaved dog. If you don't you will have a dog that knows very well he is bigger and stronger than you who believes he is in charge. You will however need a lot of patience as Mastiffs can be very difficult to train not because they are not smart but because they are very stubborn. You need to spend time and be very very patient and use lots of treats and pats and praise - and NO yelling or hitting or aggression.
You will most likely be "challenged for authority when the pup is 6 months old (especially if it is a male) and you have to nip it in the bud. Takes about 2 weeks of consistent work. The dog will refuse to do what it is told and possibly threaten you if you try to make him. You just have to make sure he has a leash on when ever he is inside so you can put him out (time out) for 5 to 10 minutes every time he won't obey without getting close enough to be threatened.
All you really need to teach is the basics - "sit", "drop", "wait", "stay" and "come". I have also taught all our dogs "manners" which means I require calm and quiet behaviour with no jumping and running around or snapping at offered treats. Our x-Mastiff who behaves like a Mastiff will still often stop and think about it before he obeys so he is always generously praised when he makes the right decision and put in time out to think about it when he makes the wrong one. He has not threatened me since he was around 6.5 months. Like most Mastiffs he can be trusted with other dogs - after a proper introduction, he is very protective of his own "pack" members, he loves cats and is very gentle with them.
A mastiff will most likely not be a pushy an obvious about getting attention as a lot of other dogs (GSDs for example) but they pats and cuddles just a much.
Just want to add that while I have never seen a Mastiff playing with a Sheltiex retriever I ahve seen a 60 kilo mastiff pup (under 18mths) playing with a 30 kilo mastiffxGSP pup, (under 12 months) a GSD pup and 2 adult Border Collies. There was a bit of shoulder barging but the more agile sheep dogs quickly learned to roll out of the way and go for the heels of the bigger dogs. Lots of fun had by all and no-one was ever hurt.
To sum up - a mastiff (including Bull Mastiffs) is far less likely to bite than terrier or other small dog, if properely socialised with a firm grounding in (very) basic obedience they are calm, dignified and quite and trustworthy dogs who are still up for a bit of fun when it is appropriate.