Hello Ryan,
Shieldbugs, as you probably already know feed, as both nymphs and adults on either sap or juices from various plants and trees or from the liquid contents of insect larvae, caterpillars etc.
Most, but not all, species hibernate during the winter months as adults, these hibernating sites can vary from under loose bark, in moss or in buildings etc.
Entry to houses is relatively simple for them, ventilator bricks, cracks in joints, roofing tiles or shingles, any small hole can be utilised by them to gain access.
Just when they emerge from hibernation, depends upon temperature, and as this can vary according to the site chosen for hibernation, they emerge often over a period of two or three months, during the spring.
If you are getting them all year round it's likely that some are landing on your roof during such times as they take flight and to avoid exposure to either predators or inclement weather they take shelter under whatever roof covering you have.
Like many other insect species thay are also attracted to light and if you have a window open at night they may well fly in.
There is nothing in a house that would naturally attract them other than what I have mentioned above. They present no danger at all to humans and they're probably just as anxious to get out of your home as you are to have them gone.
It's not a species which occurs in Britain, but it is common in southern Europe and perhaps elsewhere around the world. I see that according to various websites that it is commonly found in houses. If this is so, then it's likely that it is open windows in lit rooms at night that is attracting them into your home.
This may be a slight inconvenience to you, but far less so perhaps, than having mice take up residence.
Harry