1) She is too negative. No one likes to be around someone who is always pessimistic. Maybe the reason you no longer enjoy your best friend's company is because she drags you down with her negativity.
2) You guys don't have fun together anymore. Feeling 'bored' when hanging out with your best friend is a hugeeee red flag that you guys are growing apart! BFFs always manage to have a blast no matter what they are doing!
3) You both have different interests. You use to love all the same things, but now maybe your idea of a good time is shopping and cocktails, whereas she would rather stay in and order pizza.
4) She betrayed your trust. Maybe you caught your best friend talking to your ex or something even worse to betray your trust. You may have forgiven her, but the damage may be done to your relationship and may never be the same.
5) You don't have much in common anymore. Maybe you are in two different fields like she is a lawyer and you are working for a huge fashion firm. It can put a strain on your relationship because you are struggling to have common ground to talk about.
6) She is immature. If your BFF is still acting the same as she did when you two were 16, you might find yourself moving away from her and closer to people who are on the same maturity level as you.
7) Your friendship feels like work. If hanging out with your best friend feels like a 'task' or talking to her is putting a drain on you, it is a sure sign that you two have started drifting far apart.