I think the reasons for not chasing are as follows:
First, to a great extent, you will lose a good friend.
There are few real good friends in one's life, if you are lucky enough to get one. It's really not worth losing a good friend for his lover. The most terrible thing in life is loneliness. It's sad that people have no good friends all their lives. You never want to be alone.
Second, to truly love someone should be to make her happy. The way to love is not necessarily to be together. Love together is the most selfish. Do you really like your best friend's lover? Will you really be happy with her? There are too many problems to consider. True love, great love, should be to give each other happiness, not to be together every day and have each other. Although everyone can't be that great, I think we can try to enlighten ourselves to do so.
Third, even if you are caught, it may be a tragedy. If you go after your best friend's lover and know that she is dating someone else, it is a wrong start in itself. So what if you catch it? Even if two people are together, your love history will always be mentioned in the future, that is, digging someone else's corner. Can you stand it? If it were me, I wouldn't be able to stand it. People are awesome!