On the train to Oxford the other day, I found myself in easy listening distance of a conversation between a father and his adult daughter. "I'm a bit peeved with Sophie," said the daughter. "I'm always inviting her to things but she doesn't show up. The last time I saw her was when I catered her party. I wondered if I'd even have been there if I wasn't there as staff. I mean, I love her, and we've got 10 years of history, but she takes it for granted I'll be there for her. And always the drama with her. I could be dramatic – I'm a single mum of a toddler and a baby, but I just get on with it."

"You're right," said her dad. "Friendship has to be mutual. It's interesting to see who stays and who goes during life. I met a chap recently who's being playing bridge and going for Sunday walks with the same three school friends for 35 years. I don't have any long term friendships."

"Most of my friends don't know each other," said the daughter. "I pick them up here and there. People annoy me and I drop them. I can't be bothered. I'm intolerant of other people's standards. I don't want to negotiate. It is nice to spend time with people sometimes but most of the time I do things by myself. I like my own company."

Laying aside the ethics of transcribing a personal conversation overheard on public transport, I was fascinated by this exchange. It made me sad and caused me to reflect on the nature of friendship. Friendship can be as complicated and painful as any other inter-human relationship. But what would life be without friends? As defiantly independent as this stranger behind me on the train claimed to be, she sounded bleak and lonely.

Friendships play an absolutely central role in my life. My friends are my sanity, my spiritual lifeline, and my entertainment. They challenge me, stretch me, antagonise me (in a helpful kind of way) and cheer me onwards and upwards. Proverbs 12:26 says: "The righteous choose their friends carefully." But while we spend a lot of time thinking, writing, talking and investing in romantic relationships, friendship can get overlooked.

The novelist Hanya Yanagihara said at a recent reading in New York: "Friendship is the most underrated relationship in our lives... It remains the one relation not bound by law, blood, or money but an unspoken agreement of love." This agreement of love led Jonathan to warn David of his father's intention to kill him, despite the huge personal conflict this must have involved (1 Samuel 20). It took Ruth far from her own culture to an unknown land and an uncertain future (Ruth 1). It was why Eliphaz, Bildad, and Zophar sat by the distraught Job for seven days and nights offering wordless comfort (Job 2:13). One of the most staggering things Jesus said was that he considers us friends (John 15:15).

As with every other kind of relationship, a friendship needs tending; it needs time, care, attention and commitment. There are some stages of life when we have room for vast quantities of friends – this was one of the best things about university for me. I studied English literature and had hardly any lectures, so devoted myself to collecting people. And later, when I had small children, there was another period when space opened up for lots of deep, albeit interrupted and sleep-deprived conversation. When life becomes fuller, we need to prioritise, or risk being spread too thinly, ending up with a plethora of acquaintances – Workplace Proximity Associates, as Ron from Parks and Recreation would say – and no true friends.

So I leave you with this challenge: identify your soul mates – that handful of people God has given you to share life with. And once you've identified them, prayerfully consider what you are doing to nurture those friendships. Do you need to make more time and space for them? Are you allowing them to see the true picture of who you are? Is your relationship mutual? Friendships are not a luxury or an optional extra in a busy schedule. We need our friends. Let's look after them.

Jo Swinney is an author, speaker and editor of Preach Magazine. She has a Masters in Theology from Regent College, Vancouver, and lives in South West London with her vicar husband and their two little girls. joswinney.com