I believe that this is mostly a Catholic teaching (I could be wrong.)
For those who don’t know, they are
-Lust
-Gluttony
-Envy
-Greed
-Wrath
-Sloth
-and Pride

This is what I have to say to these sins:

-Sex is normal and necessary for life. However, overindulging in sex and abusing this gift is a sin.

-Eating is necessary for life. However, overindulging in food and drink is a sin

-Being jealous of a person is normal due to human nature. However, finding a way to obtain what others have by harming them in any way is a sin.

-Avarice is normal due to human nature. However, overindulging in the thought that one must have everything in the world for himself is a sin.

-Anger is also a normal feeling we have due to human nature. However, overindulging in the thoughts of harming others is a sin.

-Taking a break from laborious work is normal and even expected. However, overindulging in idleness is a sin.

-Being happy for your accomplishments is normal. However, to continuously brag and show off to the community what accomplishments he has done is a sin.

These sins can be called mortal because it leads the person who has one or more of these sins listed into further sin, causing the person to drift further away from God.