My lazy google search on the term identifies altruism as
"disinterested and selfless concern for the well-being of others." "some may choose to work with vulnerable elderly people out of altruism".
When it comes to the rest of animal kingdom the only kind of altruism we can observe is reciprocal altruism and this is usually an adaptive behaviour that happens to favour one specific species. For example vampire bats feed the younglings of other bats instead of their own. The bat who doesn't follow the behaviour, has her own offspring starve to death. This is as far as reciprocal altruism goes in consistent group behaviour and very few species exhibit it. Humans do not exhibit such altruistic behaviour on a group level.
Altruism in humans is rather a social perception rather than something that can be clearly defined. Most people consider altruistic acts some actions that involve helping others. This false perception occurs because we choose to focus on the act rather than the process upon the act comes to be.
Everyone has emotions, feelings, existential baggage that come into play when something is played out. If for example you grew in an environment where cats were considered pets then you will feel good taking care of them. Same applies to rats in some temples in India. Contrary In the west we kill them or feel disgusted by them because of sanitary reasons. In other words, culture is key in regards to how we perceive something upon which we act altruistically.
When we help someone we feel good because a surge of hormones along neurotransmitters flood our brain, giving us a sense of satisfaction. This is an instinctual mechanism that is also reinforced from culture. We are build with a specific blueprint to act a certain way because it benefits us as much as it benefits the group. Sometimes the scales are tipped towards our person and some other times towards the group.
A lonely lady with limited human contact might find herself drawn towards other animals such as cats. In general, pets often serve as an emotional substitute in place of human bonding. Helping a specific species of dog or cat makes you an altruist as much as spraying mosquitos makes you a killer. We can surely argue about sentience in other species but we will end up in an endless argument that will revolve around our inability to actually be other species and perceive the world through their own physiology. In other words, our altruism, yet again, is a reflection of our own feelings.
Which brings us to perhaps the most dreaded word of english dictionary: Selfishness. We are all selfish, whether we like it or not. First and foremost reason is that we aim to preserve our own self and our genes. A mother will protect her young and put herself on the line not because of altruism but because of selfishness towards preserving a fresh copy of her DNA. A hero soldier will sacrifice himself for the group because survival instincts and cultural conditioning programmed him to do so. Every single act of altruism gives us rewards one way or another and while it might be strong for some it might no be for others. Judging someone on his sense of "altruism" is similar to judging someone for being depressed. We are all different and our reactions to the environment can vary greatly based on our physiology and upbringing.
The glorification of the term altruism is nothing more than the poor understanding about how the human species operates. We will help others only when we have our asses covered. If we find ourselves going the extra mile is because of some cultural upbringing or even religious belief that aims towards a specific behaviour. On a daily basis, when we strip away the outlier cultural impact, we see individual human beings acting for their own self interest and helping others when it benefits the whole group — including ourselves. This is why many proclaim that the world has gone "corrupt". We judge others based on specific situations without examining their whole history — yet we are ready to judge ourselves positively because we are aware of our own history. The perception of altruism is nothing more than the sum of ignorance about other people's histories — a cultural narrative that aims to enhance the way we interact with each other.