We'd be super weirded out if someone in front of us in the queue in the supermarket committed suicide so we could pay faster. Apart from that probably increasing checkout times for everybody -- just imagine the chaos -- we wouldn't even appreciate "the thought", we'd be like "how DARE you use me for this?". Most of us don't mind being catered to or even pampered, but we don't want others to just throw themselves away for us. There are limits, even though it's kind of invisible most of the time, there is a line where hurting ourselves too much to help others a little bit actually hurts society, and offends others, correctly so.
Last but certainly not least: this over-the-top, dysfunctional selflessness in the sense of having no self (or rather, not respecting one's self) attracts not only knights in shining armour, but mostly baaaad types. You might say abuse breeds abuse in that someone who for some reason is playing doormat is emitting pheromones for people who like to trample on others. I really don't mean this to victim blame at all, but it's sadly true. And the less you let others violate your boundaries, the clearer your sight becomes for what you can freely give for mutual benefit. E.g. don't spend 2 weeks to save someone 5 minutes, but do spend 5 minutes to save someone 2 weeks.
TL;DR: you can't be a good friend to others without being a good friend to yourself first.