why are some bacteria considered to be "friendly" to our digestive system?

is it because they aid in the digestive process by helping digest lactose (milk sugar) and protein and helps to clean the intestinal tract, to purify the colon, and to promote regular bowel movements

http://en.wikipedia.org/wiki/Gut_flora

read the introductory paragraphs.
Beneficial is a much better word than friendly.

Yes, that's correct! Some bacteria are considered "friendly" or beneficial to our digestive system because they provide various advantages:

1. Aid in digestion: These bacteria help break down substances such as lactose (milk sugar) and proteins that our bodies might otherwise struggle to digest efficiently. For example, lactose-intolerant individuals have difficulty digesting lactose, but specific friendly bacteria can help metabolize lactose, making it easier to digest.

2. Gut health: Friendly bacteria, particularly those in the large intestine, contribute to the overall health of our gut. They play a crucial role in maintaining the balance of microorganisms in the gut, known as the gut microbiota or gut flora. This balance is important for proper digestion, nutrient absorption, and immune function.

3. Improved bowel movements: Certain friendly bacteria produce substances like short-chain fatty acids, which promote regular bowel movements and help maintain healthy function of the colon. These fatty acids can also provide nourishment to the cells lining the colon, keeping them healthy.

4. Protection against harmful bacteria: Friendly bacteria can help prevent the growth of harmful or pathogenic bacteria in the digestive system. This is achieved through various mechanisms, such as competing for nutrients or producing antimicrobial substances that inhibit the growth of pathogens.

When considering the "friendliness" of bacteria, it's important to note that each person's microbiota composition can differ. What may be beneficial for one person's digestive system may not have the same effect on another's. Nevertheless, maintaining a healthy balance of friendly bacteria is generally associated with improved digestive health.